I'm not exactly sure what a magazine template would be since there are so many magazines out in the world, and most of them try to look different from everyone else.
An InDesign template, in and of itself, is not something mysterious and complex that you need to buy. It's an ordinary file with the .indt extension that triggers InDesign to open it as an untitled copy of the original rather than overwriting or changing the template, so you can make your own if you like.
Here are some of the things that you would normally include in your template: Page size and orientation; Master Pages with various column structures predefined, and perhaps placeholder frames for text or images to keep a consistent layout grid from page to page, and colored background or accent elements if you use them and they need to be on more than one page; predefined layers to help keep you organized; color swatches for the color scheme you've adopted, if any; paragraph, character, and object styles to keep the look consistent.
Yes, you can change the document setup on your existing template if you like. You said some items were leaving the page -- no big surprise since the new size is shorter. It will help, but is not a panacea, to go to Layout > Layout Adjustment... and turn on Layout Adjustment, but you must do it BEFORE you make changes. Frames that are snapped to guides should resize or move, but there is almost always work to be done after this kind of a major layout change. You should keep a wary eye out for overset text or threaded text that has reflowed and moved something to a different page from where you intended, and for images that have become distorted, and of course things not snapped to guides are probably all now the wrong shape or in the wrong place.
I think it might be easier to start over and make your own template from scratch, using the one you have as a guide for relative positioning, and you can even import the styles if you like them. File > New > Document... and set up the page size (letter) and orientation, and the margins and columns which will appear on Master Page A (you can add other master pages later). Press the More Options button if the bleed fields are not showing and enter .125 in (you need the units if you are not set for inches by default) for the bleed. Press OK
If you want to import the styles from your current template, open the Paragraph Styles panel, and from the panel flyout menu choose Load All Text Styles... then in the dialog that pops up navigate to your existing template and open it. For conflicts, choose incoming styles to use the ones from the template and redefine the [Basic Paragraph] style that is automatically made in every new document. Open the Object Styles panel and its flyout menu to choose Load Object Styles and do the same thing.
If your original template has custom swatches you want to use, there are a number of ways to get them into the new file, but one of the easiest is to simply copy an object that uses the swatch and past it into the template. You can immediately delete it if all you want is the swatch and not the object itself.
When you are done doing all of the the tings that you need (adding master pages and setting up placeholder frames, etc.) Save AS and change the type to InDesign Template in the type dropdown. Store it anywhere you like. To use it, just go to File > Open... and choose it and you'll get a clean untitled copy. If you need to make a change to the template that should become permanent and carry forward into new copies, File > Open... and find your Template, but before opening, change to Open As Original in the lower left corner of the dialog. Make your changes, then save and close before you use the template for a new document again.
All of that probably sounds a bit daunting since you've never used InDesign before. ID is NOT a simple and intuitive program and it isn't like working in your word processer. Magazines are complex projects, and I sense you probably don't have a lot of experience using other layout applications, either. You should really invest a little time in doing some training. The most commonly recommended book for new users here on the forum is Sandee Cohen's Visual QuickStart Guide to InDesign. More advanced users will find Real World InDesign by Blatner, et. al. a good choice. The vido tutorials at Lynda.com are among the best, and you'll also find a lot of others by googling. Sites like InDesignSecrets.com and TheInDesigner.com may be of interest. You should spend some time with the online version of the help files as they often contian comments from users about errors and omissions, and of course we're here on the forum to assist when you have questions.
Thank you very much Peter. I bought this template: http://graphicriver.net/item/stylish-indesign-magazine-template/264182.
While I am new to InDesign, I am not new to similar software and was able to start using it immediately. While I know how to use such software, I am not a talented designer and to create a magazine that looks that good is something I have been unable to do and the template instantly took care of so many things for us. I expected what you have told me was the case. Every template I have found is A4 size with the exception of a few ad page templates so I thought I would ask. I very much appreciate your response. Thank you.