Framesmith DC at 14th and Q is great, and I’ve always found them to be reasonably priced. It’s by appointment only, but she’s around most weekends so it’s not usually an issue. Framesmith has amazing sales during the “dog days” festival on 14th St if you can wait until August.
I’ve also tried the frame store on 18th St in Adams Morgan, but wouldn’t recommend them. The quality is fine, but the prices are random and occasionally outrageous.