SO many people don't understand that pressure and flow are two independent measurements. Congratulations on making the smart insight that measuring flow is exactly what you need to do!
eBay is your friend on this one. New meters in that flow range can top $1000 each. Here is one listed for under $10!
2) DWYER AIR FLOWMETER FLOW METER SCFH 0-600 RMB-57-SSV | eBay