Well ... type two spaces in the Find Text field and enter a Tab code in the Replace With field (which is "^t"). That ought to do it, unless you are not sure if all of your double spaces should be replaced?
This is where your good friend the Edit>Find/Change... menu command comes in ...
- Highlight all your index text where you want to make the substitution. This precludes InDesign from making changes where you don't want them made.
- Select the Edit>Find/Change... menu command.
This opens the Find/Change dialog box. From here, it's as simple as 1-2-3:
- Go to the Search: options box and choose Selection.
- In the Find What: edit box, type two spaces. Go to the @ button to the right of the Change To: edit box and select Tab. You'll see that it automatically puts the characters ^t in the edit box. That's InDesign data-speak for Tab.
- Click the Change All button and it will change all the double-spaces to tabs and your dot leaders will shine through and align the page numbers with the tab you've got set up in your index. Then click the Done button to get back to your updated InDesign document.
1-2-3. Done Deal.
Thanks to all, I used Jongware's solution because it was the first.