there are two main sizes for bongs, but by that i mean, there are two main used sizes of bowl/slide , just take either peice [bowl or slide] to a headshop and ask for a replacment they'll be able to help you out...

i made the mistake once of walking into a headshop assuming all peices are teh same.... they aren't.... just make sure to take on or the other so they ca properly match up your peice...