That's a great question, and is getting much tougher to answer...
I always use the sectional frames that you put together yourself. Then you just have to cut the mat/glass/backing to the same size. It saves about $250 - $300 per framing job.... leaving lots more money for new pennants
In the past, these were available at Aaron Bros, Michael's, Ben Franklin, Value Craft, and even Joanne Fabrics. I think all of them have now discontinued these sectional frames. I was told they weren't great sellers. It's either that, or perhaps they just prefer to ream you for custom framing.
Maybe try Hobby Lobby.... they're not readily available to me (here in the NW), but perhaps there's one near you, that you can check out.
If that fails, you can try Amazon. Good luck!