Looking for a good online vendor for picture mats. Who have you liked?

I want to display some fabric art pieces (called molas) which I think I should have a double mat due to their surface texture. The ideal vendor would not only offer good value and good color options, but provide an easy to use online tool for visualizing a piece in the various mat color combinations they sell.

(This photo is not my piece, but is provided as an example of a mola with a double mat.)