I have a couple of Makarovs and I go through a lot of 9x18. It's not all the available around here. Bron's in Sturbridge usually has some and I've seen a small amount being sold at Four Seasons and Zenks. Though I think I bought out all that Zenks had a while ago and am not sure if he replaced it.
The only place I've found it on a regular basis is at Northeast Trading Company. It all depends upon how you feel about them. The cost has been going up regularly. I used to get it for $7 a box of 50, but last I checked it was going for around $13. The NY guys at the gun shows had it in Marlboro last time around for $11 a box, which I thought was a half way decent deal.