They can be a bit hard to track down yes, but a pair would be a good choice for your tank
Mike's Rifts sometimes have them, though not right now, but it's worth keeping an eye on his stock list as his Tangs are excellent.
Your LFS may be able to order them for you if they don't have them in stock; it's always worth asking. It might also be worth posting on the BCA site in the "Looking For" forum (scroll down), to see if anyone has some for sale.
Escargot shells should be fine as long as some are large enough for adult females to get inside if they need to escape the males. Dwarf Sumbu aren't true shell-dwellers as such; they tend not to actually live in the shells, but use the shells for spawning and rock crevices/caves as homes.
Managed to source some Dwarf Comp. from Wildwoods but gonna take a couple of weeks to get them in. If i were to get 3-4 fish and a couple paired off,would i have to remove the others or does it depend on the sex of the other fish ?
It's not sex dependent unfortunately. In a larger tank you'd maybe be OK as the Dwarf Comps are pretty mild-mannered as Tangs go. Given the small footprint of yours I think you'll have to rehome the spares if you get a pair as once they start to breed they'll become much more territorial and there's not room for separate territories for the unpaired fish.