If you purchase the Plus subscription to Photoshop.com, you will find a few more available.
I know of nowhere you can get additional themes.
But many of the plug-ins for Premiere Pro also work in Premiere Elements. Check out Burger Transitions for some you can download.
My "Hot Tips & Cool Tricks for Adobe Premiere Elements" book also includes instructions for creating custom transitions using keyframed effects, presets and the Custom Wipe transition.
About the Plus subscription at Photoshop.com, I have the impression that it is available only for US customers. Correct?
Welcome to the forum.
There are several suppliers of Transitions (and Effects), that will work in PrE. They are not freeware.
Check out New Blue FX, and also ProDAD's Heroglyph and Adorage. Also, their Vitascene is great for some neat Effects. Also, Pixelan's Spice is a great Transition provider. Not sure how well that program might interface with PrE, but works great in PrPro.
PS - I thought there were a few Themes at Muvipix, but since Steve G. did not mention that site, I assume that I mis-remembered.
I think you are right, sfilgr. It is only available in the US.
Here's the Burgers Transition site.
And of course my book is available on Amazon.com as well as through the Muvipix site, if you'd prefer to make your own.