search google for high pressure check valves. there are a couple i found, but i couldn't find prices. the inline "soft seat" type valves will probably be the cheapest. good luck.
You should be able to use a check valve from a hydraulic application.... I know your using air vs fluid but I've used hydraulic stuff for all kinds of things including air.
They should be a cheap solution.
2010 BMW 335D
1994 Opel Calibra 4X4 turbo ( C20LET 2.0L Turbo )
2002 Daewoo lanos