The "official" seal kits are sold by Hasselblad or through Hasselblad repair shops.
They are also offered on eBay and such, but i suspect (never bought those from there) they are DIY-products.
I use sheets of black closed cell foam material sold at crafts/hobby shops. I forget how thick exactly, but about 3 mm.
They are cheap (a few $), and large enough to cut many, many pads out of.
I don't know how long they last, since they are so cheap that i replace seals probably much more frequently than needed. But it's obviously better to do it too often than to wait until you get frames ruined by light leaks. (I keep track of when what back got a new seal, which at least told me when a back was about to be overdue.)
The original foam pads have a mylar backing, with a curve in it that pushes it upwards. The foam i use doesn't, but is a bit thicker, so still seals he slot completely. But you notice that inserting the slide takes tiny bit more 'push'.
The foam is wrapped in a mylar foil, that protects it from the sharp edge of the dark slide. That foil doesn't need to be replaced often. Only when it is worn through. The foam pads themselves however deteriorate, losing elasticity fist, and ending up as a sticky goo.
I'm still looking for a replacement for the official foils (still have a small supply of original ones). I believe some people cut them from exposed and developed film leaders. Haven't tried that myself yet.