For the "handy" artist -- you know who you are -- this site might be interesting,
Framing4Yourself .
You can purchase a variety of finished mouldings in both stock and custom lengths. What I was particularly glad to find here was a variety of raw (unfinished) picture frame mouldings in various lengths, like that shown below.
If you don't happen to have frame-building hardware (mitre saw, web clamp, v-nail driver, etc.), then you'll opt for the custom lengths, which come with corner joiners that quickly and easily snap in to complete your frame.
Another similar source is
FrameFit , though the last time I used them, they did not carry raw mouldings.