The stock tips are just held in place with two bolts and are not connected to the exhaust pipe at all. I have both of mine pulled of right now (after one boke off on a rough gravel road). You shaould be able to bolt on most aftermarket tips right to your tail pipe. The only thing that might be a pain is, when I looked at my exhaust, the pipes comming out are bent to an oval at the end so you may either have to bend it back some or make sure you get a tip with an inlet large enough to fit around it.
As far as where to get them, there are so many places you can buy them that the best thing I can say is to find a style you like and then just shop around for them. I have seen the selection at Pepboys and other parts stores and their selectins generaly suck!
I know for my car ('95 SSEi) I like the 3' double square chrome tips. They look like this:
I figure they will fill up the rectangular stock openings quite nicely.