Can you obtain income approval for a retirement visa in your home country before moving to Thailand?

You appear to be an American? If you are you can't get income approval at all as your embassy won't issue it. You have some choices though

1. Get a Non-OA visa before leaving, this is good for 12 months and doesn't require showing any income in Thailand (but does need insurance)

2. Get a Non-O visa before you leave (might need insurance) which is good for 90 days. To extend inside Thailand you need to transfer 800k baht into a Thai bank account in your name 2 months before applying for an extension

3. Come on a tourist visa, open a bank account in your name, transfer 800k baht into it from overseas and apply for a Non-O visa inside Thailand. The first extension is as 2 above

Subsequent extensions beyond the first year can be obtained by transferring a minimum of 65k baht into your bank account each and every month for a full 12 months before applying for the extension

If you mean the one for in-country visa and 1-year extension, you need an affidavit from your embassy in Thailand.

Now, it's between you and your embassy if you can apply for an affidavit when you are still in your home country or you need to do it once you arrive Thailand. Contact your embassy in Thailand.