When I was prototyping I just used these ones since I could get them next day.
http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&KeyWords=294-1100-ND
Two of them are enough to cover all 4 phases.

You can get heatsinks cheaper than that if you don't mind waiting though. I bought these guys as well, and just cut them in half and put both halves on the board.
http://www.ebay.com/itm/Aluminum-Heat-Sink-10pcs-lot-x-150-20-6mm-for-LED-Power-IC-PBC-Module-Transistor-/181132035289?ssPageName=ADME:L:OC:US:3160