dunno if there still around, but when i had an a5, seemed like dead on paintball was the best place to buy tippy ups from, have a look.
probs best to get an egrip first though.
and with barrels, like ajermstad said, when you but one, you specify what thread you want (most guns are different). id recomend that you had a fairly short barrel on a 98 though, its long enough as it is.