Wally World (WalMart) usually has marine sealant here in Pa. - So I'm sure they have it in all stores.
When I purchased my latest boat (used) it was missing the electronics but had the transducers still attached to the transom. I added all Garmin electronics, and of course the bolt patterns were different. I took stainless screws (just slightly larger then the holes), put sealant all over the threads and ran the screws into the old holes. That was 5 years ago - no leaks yet.
Did the same thing when mounting the new transducers. Cover the threads in sealant and run 'em in. This will build up under the bolt/screw heads like a washer would. I knew it was a complete seal by the amount of sealant that was on the inside of the boat as well. (not nearly as much, but it should still track in)
I want to say I used a 3M product of marine sealant - white. Can't recall the exact name on the tube anymore. (squeeze tube)