Welcome to the forum!
I agree with what others have said about just calling some barns and asking about rider weight limits. I'm sure you'll find some are more accommodating than others, but you will find one!
If you want to start preparing yourself for riding, then I would start with some simple exercises and stretches--someone mentioned resistance bands. That would be a great start. Go on some walks, starting with very short distances first and slowly building up to longer ones or slowly increasing your speed. Don't overdo it and hurt yourself, but I think you'll find it very helpful if you're working on your leg muscles already when you get back in the saddle again.
Good luck, and have a great time riding!