You can DIY one from pretty much most 2d look down transducers. Two things to keep in mind, you want a good transducer with enough juice, and you want to mount it so it faces straight down while on plane where it won't "see" much turbulence or bubbles, and you want to mount it where it will be in contact with the same surface opposite the water with no gaps or foam in between, as far as mounting it, you can glue it with epoxy, or sit it in a bath of oil, I guess that's more than 2 things...
There are also true thru hulls, but drilling a big hole on the bottom of your boat takes some fortitude. Still have the "hockey puck" from the 26 Regulator, couldn't do it to the 23 Seacraft, went the shoot thru route on her, God I miss that boat.