Where can I purchase Instant Yeast?
It's what I use exclusively and I've never been able to find it locally--looked everywhere. I buy it online in 1lb. bricks. On the bright side, it keeps just fine in the fridge and I actually do end up using that much in a year. If you're in a pinch, know that there's really no absolute need to use it other than the convenience of not having to pre-dissolve in liquid. You can sub in active dry just fine if you're willing to take that extra step.
Depending on the brand, instant yeast is the same as bread machine yeast. That you can usually find at any grocery store, but YMMV. I don't buy it this way because what I find is usually out of date or about to be and it's MUCH cheaper to buy the 1lb. bricks.