I had the same question. I used a "FH-M525A Freewheel Body Unit" from Shimano (UPC: 689228017450). I had to redo the hub spacing, so an axle spacer kit is necessary (if I remember correctly, I moved the biggest NDS spacer to the DS under the freehub and put a 3 or 4mm spacer on the NDS, just make sure there is about 5mm of exposed axle on each side to fit in the dropouts). The wheel did need a slight re-dish, but it wasn't too bad with proper hub spacing, just add lubricant to all the spoke nipples before starting. I am currently running SRAM GX 10 speed and it works great.