I'd highly recommend Ronco for the new holding tank. http://www.ronco-plastics.net/
*Reasonably priced, VERY sturdy, easy guys to work with. They have hundreds of stock sizes. I had an inspection port put in mine, and all the penetrations are through the top. Two outlets... one to the pumpout fitting, one to the overboard diaphragm pump. That way, no need for a Y valve. The pumpout fittings include dip tubes to reach the bottom of the tank.
I'd also suggest putting a big one back in. You can never have too much holding tank capacity, and it'll be something folks will look at when you sell your boat someday.