You can get brass sheet from just about any hobby store, but 0.025 (in reality, it's probably 22 gauge, which is 0.0253") might be a touch too thick to fit. The necessary thickness is a bit less, ~0.0236". 24 gauge (0.0201") might work better, since the clamp is made to go smaller than the nominal size, but the seat tube can't get any bigger.