Below are some links to stores that sell frames and Mat’s online. Locally I use Blick, Michaels or Hobby Lobby (With coupons). Of course there are many local framing shops that do excellent work.
Fort Repetition and Oak Avenue
To fit an 18x24 frame with an 11¾" × 17¾" opening in the Mat. Try Blick Art Supply.
I like this in Blick Manor White (17220-1339)
You can also buy frames online, I like this image in a sleek black frame.
I See You
to fit a 12x12 Frame with a 0x0 opening in the Mat.