I've always just rigged one up with whatever was laying around.
If you have a really rigid and large plastic container that has a air tight cap, you can build one to suit your needs.
2 vinyl hoses go into the cap (via holes you drill) -- one only goes in a few inches -- this one you will connect to the shop vac hose end (either with tape or just hold it by hand)
the other hose goes deeper into the container, and is typically a smaller diameter and longer. this is your suction end that you stick into whatever needs evacuation.
Once you start up the shop vac, it will pull a vacuum on the container and you'll get suction on the other hose. Since the hose connected to the shop vac is much higher up, you aren't sucking in any of the fluid, as long as you don't let the fluid level get too high.