Tom,
Ebay has double hose regs on there quite often. But if you want to get into DH diving, go to Vintage Double Hose, in the store to the regulators. Bryan has some "get started" offers that just can't be beat! For $250 you get a rebuilt (with all new parts) DA Aqua Master, banjo fitting, everything you need to get going. And it comes with a one year warranty! Get that outfit from Bryan and all you have to do is take it out of the box, pop it onto a reg and splash! It's a great deal.
Buying from Ebay, you'll probably pay upwards of $100 just for the reg, then have to spend more money to get it serviced and buy the accessories like the banjo fitting, hookah port adapter, maybe new hoses, duckbill, mouthpiece valves, etc. Get Bryan's deal and you get a reg that's ready to go right out of the box.