Here's a very good deal on check valves. Each one includes two Fernco-type rubber couplers, and SS clamps! That's 10 valves, 20 Ferncos and 40 SS clamps, for $60 and free shipping.
EDIT: For some reason he's listing these one at a time. If the listing is sold, look for other items by the same seller, he's probably relisted it.
For reference, I have used these valves to eliminate the manual operated ball valves on gasifier inlet, blower outlet, air filter lines, and pressure relief valves underhood. They work awesome, I've never had one get stuck or leak.