I got mine from Junkyspot (the UV cut!) but they're currently out of stock... (so my last seal jobs were done using Testors... it works okay and I haven't had issues with it yet, but I do know a lot of people have issues with the sealer itself yellowing.
Hobby shops are hit and miss-- mostly they just have Testors, but places with a lot of import kits sometimes also have MSC.
You can sometimes find it on Amazon, though-- just while researching the question of where to go when Junkyspot is out. If you search Mister Super Clear, you get a few results. (Junkyspot was cheaper... but a little more hunting around might find you a better price than the first result I pulled up-- at Junkyspot I think I paid thirteen or fourteen dollars, on Amazon I'm seeing the same can going for about twenty bucks.)
PlaJapan sells it for about fourteen/fifteen bucks (the matte, not the UV cut).
And ThinkPink is a European doll/hobby seller that has it for thirteen-fourteen euros and change depending on whether you want matte or UV.