FWIW, I have used www.pastacheese.com
but not for impastata. They are located in Garden City, NY which for me living in NJ equates to getting my cheese usually within a day or two of ordering.
I agree with Mark, the cheesecloth trick in the fridge is good, especially if you don't want to shell out the cash for the more expensive impastata. I also love ricotta salata and would definitely try it on pizza. This should be easy enough to find in the supermarket.
I have seen impastata in RD but don't know the price. The other thing you can do is to check your local Italian Deli, most of them carry those specialty type items in their refrigerated section. This actually might be a better option since most of us don't need such huge containers of cheese that you would normally see in RD
. Good luck.