They are standardized but you'll probably be able to find a higher quality membrane for less through an aquarium place. I did a quick look and Home Depot carries a 24gpd membrane for $75
http://www.homedepot.ca/product/r-o-...embrane/923084 J&L also sells a 24gpd membrane for $59.95
http://www.jlaquatics.com/product/ro...29+Filter.html If you need a higher flow membrane like a 75gpd or 100gpd you may not be able to find it at a hardware store. If you order a few items to split up the shipping, the prices on Bulk Reef Supply are even better.