These bulbs can be usually found at Wallymart. I
picked up 3 pairs a few months ago. They were
around $3 per pair I believe.
Wallymart also carries the 194 bulb of this kind as
well-- ones for the license plate size. You should
check there before paying double on ebay.
As far as 3156 and 3157, I don't think it matters
much. I'm using 3156 (X comes with 3157?), and
they work fine. Could be wrong but I believe the
numbers have something to do with the brightness
of the bulb??