If you want quality custom frames use Frankenframes.com I have ordered tens of thousands of $ in frames from them for my gallery work over the past 20 years. Consistent quality.
Essentially - around here - you have two choices - Michael's or the local framing gallery. Otherwise - it's on-line. Sometimes you can find decent frames at thrift stores - but usually - because they don't care or someone didn't care - they are marred and practically no good, and trying to find a specific size - plan on spending lots of time - and time - well - it's money.
Would you believe Ikea? but then again if it is for something special and not a gallery hanging I too frequent the thrift shops including Goodwill..awesome finds...
I like cheap frames, and thus like the cheap plastic frames at Arron Brothes, during their BOGO1ct sales.
Also, a miter saw and some millwork or weathered wood can be very useful in making your own. Good luck finding the weathered wood.
No 20x30 frames at my Michaels but they will make one for a (very high) price. That size I usually order. Goodwill rarely has anything good, but it is nearby and always worth a look. Local framers are superb, but again, expensive.