http://www.ebay.com/itm/Sony-MH1c-Headset-SmartKey-Control-for-Xperia-Bulk-Packing-Black-/170876113045?pt=PDA_Accessories&hash=item27c9029095
I know this one can ship almost anywhere, but it's 2x the price
http://www.smartphonebay.net/Sony-Ericsson-Sony-Xperia-MH1C-Handsfree-Headset-p/hds-semh1c.htm
I have never bought from here, but it looks like they have the lowest price.
After people got hyped on the MH1C, the price appreciated from $20 to >$35. neither amazon nor its sellers even have the bulk version anymore. These links were the cheapest I could scavenge, but the price should be around $20 on the FS forums