Welcome
@TheCreekKid 
That really is an unusual size. First of all, do all of your seedlings need to be on a heat mat? I usually only use heat mat for peppers and eggplant, and a few herbs. Tomatoes, herbs and many other veggies are just fine without heat for germination, unless you are trying to start seedlings in a cold basement or similar. (Just FYI-- I grow about 1200 seedlings per year for our master gardeners group).
You might try checking some of the websites geared toward more commercial growers (e.g. Greenhouse Megastore). Otherwise, you might find it more economical to use 3 10x20" mats. It appears your unit is 30" wide, so 3 10" mats would work (or 1 20x20" and 1 10x20"). Not a big deal if it hangs a few inches off due to the extra length of your mats.
Good luck with your search! I have two of the Gardener's Supply shelves/light units and they've held up really well for a lot of years.
