Certainly not heavy duty, but I once had a project with a need for one-way valves and I ordered these
. Kind of like what a swimming pool air mattress uses.
If you're talking about those big double seal jobs they use for inflatable beds, you can often find those in outdoor shops. Heck, I think I even saw one at a Walmart/Kmart/some kind of 'mart'. A quick google search should find a plethora of those.