I found a grossly oversized Hx here:
copper refrigerant pipe: $113
TURBOTEC | COILS, STEAM & WATER | 3-1/2 TON COAX COIL | Surplus City Liquidators
cupronickel refrigerant pipe: $208
TURBOTEC | COILS, STEAM & WATER | 4 TON COAX COIL | Surplus City Liquidators
Both are rated at 450 PSI working pressure (burst pressure much higher). Your water-source setup should run at much lower working pressure in cooling mode (450 PSI = 125 degF saturation temp, 100 degF Tsat = 317 PSI). In heating mode, pressure is not an issue (freeze-up is related to pump sizing).
OR
You could look into China and have one custom built for cheap. Some suppliers will pretty much design and build a single one-off part for you, given pertinent performance specs. Not very expensive, either.
http://www.alibaba.com/product-gs/15...Exchanger.html