1) Is there a place where you can buy them ?
2) If not, are they a common drop ?
3) Can you bring them to the other dimensions ? (edit : using cheats like removing/adding a char with the items *isn't* an option)
4) Is it possible to use them during the 3rd phase of the final battle ?

(inventory is a bit bloated at the moment, so I'm trying to free up space and wondering if I can safely drop the ones I have)

1) Not that I know of
2) Not really
3) Nope
4) Halas, nope : Not a single Potion of Speed in Bane's Dimension.