Just about everyone sells them, the price varies quite a bit betwen models and sizes. Check out www.cabelas.com www.boatus.com
. That will give you an idea of whats easily available. There are also setups out there for simply hanging over the side of the boat, some are like nets or bags, others more like barrells with lots of holes drilled in them.
Some of the prices are ridiculous for what they are selling ($250 for just a tank if you find the true rip-offs). Some are more in the $70 range for an equivlant size.
It's pretty easy to put one together yourself though, and it would cost much less. No matter what, you need to keep in mind where the weight is going (depending on how big the tank is).
A plastic barrel w/ a retro-fit lid, cut to the right height works great for a very small investment.. for something really custom, plywood+fiberglass+epoxy tanks are fairly easy to build and can be made to any shape.
Two things to keep in mind:
1. The tank needs to be round or oval; baitfish are dumb, they get stuck in corners and die.
2. You will want the tank to be flow-through, this can be accomplished by using two pumps and a float switch or by having a drain near the top of the tank that can let the water drain down and out of the boat. The downside to using gravity is the tank must be well above the waterline, bringing the center of gravity up.. the upside is it is much simpler.
So, aquire the tank that is right for you, your boat and budget. As far as balance in a 17 ft CC, just ahead of the CC might be a good place to mount it. They sell livewell/seat/cooler type setups as well that could replace or fit under the seat; there is allways space in the stern too, but depending on how much weight you are looking at, the stern tends to be pretty heavy to begin with.
For a flow-through mount that will conserve power and only require's one pump: Use an aeretor head or though-hull at the end of a "pump-in" line to fill the tank. If it's pumping in from the bottom use a one way to keep it from draining out. At the top of the tank, drill a hole and use a through-hull fitting to attach a hose; at the other end of the hose, use a fitting & a one-way valve (for safety if it's going through the hull of the boat) to route the water out of the hull. Keep in mind that this setup is using only gravity and the top of the tank needs to be well about the waterline. You could add a second pump to aerate, but it's probably not neccesary with constant exchange of water.
Some setups just let the tank overflow onto the floor and the water gets bailed by a self-bailing ****pit; others have the tank hanging over the transom.
I built a tank below the waterline and placed a float switch at the top of the tank. The tank is constantly filled, when it gets near the top, another pump kicks on and empties it a bit. This setup works fine, but two pumps are needed, and the pump-out must be a stronger pump than the pump-in. It's nice because it fits into the floor and keeps the weight low.
As far as the salt h20 pickup, I have never really liked the idea of those. Any time you are punching hole below the waterline, it just seems to be asking for trouble. It's pretty easy to just mount a washdown or large bilge pump to the transom, below the waterline, and then run a hose up and into the boat.