my '88 CRX Si has a big hole rusted thru the resonator Do I really need it or can I just have a straight pipe put in ?

You don't need it. It will be a tad bit louder without one, but shouldn't be too noticeable.

just out of curiosity, where can you even get one ?

Ok, I have found a few but my car is in the shop right now so I can't measure this myself .... what size pipe does an '88 CRX Si have and what is the length of the resonator so I can order one now.