I haven't ordered from them personally, but I remember hearing good things about vt4c.com for silver wire. It's a heck of a lot cheaper than HGA (although HGA used to have some outrageously good sales every now and then, I don't know if they still do), but they are located overseas so it could take a while.
Edit: And to answer your latest question, I've gotten nylon multifilament from cableorganizer.com and wirecare.com, both seem pretty good. You'll need nylon multifilament if you want to reduce microphonics, TechFlex will probably exacerbate the problem. Also keep in mind that nylon doesn't expand very much (I've had good success with 1/8th nylon for sleeving 3-4 wire braids), whereas TechFlex can expand a ton. If I understand the last question, I would advice against Techflex over nylon over wire, since that would lead to one stiff cable (chubby enough for interconnects, likely unbearable for a headphone cable).