I've got lucky a few times at Hobby Lobby and even Michaels with standard size frames but for anything custom I use Frame Destination and absolutely love them.
I also recommend Frame Destination and specifically matte black, Nielsen P15 aluminum channel frames. They are sturdy enough for up to about 20x30 yet narrow and unobtrusive. You can buy similar frames elsewhere for a bit lower price, but I have stuck with FD because they cut the frames precisely and the corners look perfect. FD is also a good choice for mats and foam core backers. I have also bought these from Michaels and Blick when on sale with free shipping.
I recommend spending a $100 or so for a Logan mat cutter and cut all you own mats and backer boards. You will save a lot of money and you can custom cut the opening size if needed. I typically compose in camera and use a standard 2:3 aspect ratio which allows me to reuse mats. I typically print either 12x18 on 13x19 paper or 16x24 on 17x25 paper and use final frame sizes of 16x22 and 20x28.
