craigslist is typically your best bet, but they don't come up too often. Unless you've updated your boat - looks like you're running a DD from your profile, you shouldn't have too much trouble finding a used pylon mount (V-drive booms, however, are harder to find). I believe the SN's of that time-frame had straight pylons, and as such you could use either a contour boom or straight boom. If you have a tapered pylon, you're stuck looking for a contoured boom. Sometimes folks will list them on their own, but you can also find them in boat for sale ads where they intend to let them go with the boat, but if you ask, they may sell it separate from the boat (I did this last year - picked one up for ~$100).
I'd shoot for a pylon-mounted boom over a tower boom. They're easier to set-up, and also easier to adjust for different skier heights.