In local shops I usually have to pay about €2.50 for 3 sheets of waterproof sanding paper. Too expensive imo and I have been looking for a way to save money. I tried cheap sanding paper from Toolstation once but that wasn't suited for lapping stones. Today I ordered 15x5 sheets of Norton sanding paper from Robbelien.nl (120/180/320/600/1000). At only €0.22 excl. per sheet with €2.75 excl. shipping cost

I bought that many sheets because I was expecting having to pay €7.50 for shipping