I've only ever found them through Advance Auto and Rock Auto, as noted above.
I haven't changed mine, so I've never had to actually buy them, but I do recall one of the re-manufactured sides being 'lower quality' than the other judging by the reviews on their site (Advance). This may have been corrected, by now, as the last time I looked into them was like two years ago.
I'm sure the non-reman ones on Advance are fine. Just make sure it's SVT specific, as teh spline count is different.