Any recommendations for finding an apartment for seniors that rent is based on income in Delaware county, PA?

Just contact your local dept of public housing. That’s where you would go in PA to apply for housing assistance & they will have a list of all their housing complexes and be able to tell you if there are waitlists and get you on a waitlist if necessary.

Try these people:

https://www.voa.org/older-adults

Go on down this page about halfway and you'll see a section subhead: AFFORDABLE HOUSING FOR SENIORS. There's a link right there that will explain how it works, and as far as I can tell they are active in your area.