Hi Deanna,

welcome to Typophile!

First of all: you posted in the ‘Typographic Education’ forum. On typophile, there are several forums. I think this topic would rather fit into the Design board, as it doesn’t deal with educational concerns.

What kind of application are you looking for? Newspaper fonts, (urban) display fonts, book typography?
What place and culture, United States? Prohibition and Al Capone? Jazz, Swing, Broadway? Art Deco, Art Nouveau or expressionism?
Narrowing this down might help us.

You can always browse font vendor sites for keywords, like:
http://www.myfonts.com/browse/keyword/1920s/

F

wikipedia will prove useful.

Many of our modern typefaces were designed earlier than we thought they were.

I believe:

Gill Sans
Futura
*possibly* Cooper

were designed in the 1920's, although they didn't gain momentum till they were used for a while.

If you're looking for typefaces that were popular in the 1920's, you'd have to look for typefaces that were designed much much earlier. Due to the technology of the time, they could make for quick and easy distribution like we can with digital downloads the day after they're created.