I have never been able to find a ready-made frame that will fit these perfectly, but I have found some Pottery Barn frames that will work nicely if you get a mat custom cut to fit the print.
FIRST, BUY A READY-MADE FRAME FROM POTTERY BARN
Pottery Barn has some lovely ready-made wooden frames that are reasonably priced and just the right size for framing my 16x16" prints. The only problem is that the mats that are included don't have the right size opening, so you will need to get a custom mat. The two frames I'm referring to are right here:
25.5 x 25.5" square frame with multiple openings (the 1st frame on the page)
25 x 25" square frame with oversized matt (the second to last frame on the page - says 8x10" but that's referring to the print size)
THEN, TAKE A TRIP DOWN TO YOUR LOCAL ART SUPPLY SHOP
After you have your print & frame in hand, take them down to your local art shop like Michaels or AC Moore and have them custom cut a mat and assemble it for you. It will be much more affordable than having a professional frame shop custom frame it for you.