Most luthiery suppliers should have a good selection of different fretwire styles - but you'll probably have to decide which one is the best match for what you want. I didn't find anything labelled "Yamaha fretwire" in a quick look.

You can try the front door at Luthiers Mercantile International, or go direct to Fretwire for what's available at LMI.

I've done some business with LSI, and they've been helpful.

The other "name" in the business is http://www.stewmac.com/cgi-bin/hazel.cgi?action=SERVE&ITEM=catalog/s.html&1=%2Bsection%3DFretting_supplies&5=%2Bsort%3D1>StewMac. I haven't dealt with them, but they've been recommended in a number of other threads.

John