Try your local hardware store. Look for "Grant's Stump Remover". It is 100% KNO3. It goes for about $5 a pound.
Thre are other stump removal brands, but I think that only Grants is KNO3.
You could try a agricultural fertilizer distributor as well. You would be looking for "13-0-46 " . They might call it KNO3 as well. If you can get it there, a 50# bag should run you under $20. My local delaler (a big place) does not carry it any more, but I hear that some places do. My local dealer only sells 500# of anything minimum buy... sort of commercial sales only.
I have seen it on internet but shipping is a killer.
The $5 bottle of Grants will make a lot of slow match.
As a side note, I add food coloring to my mix. It colors the rope so you know it is slowmatch and also gives you a hint as to how uniform your job came out.... and it looks sooo pretty as well
Make sure you don't have a nylon core in the cotton rope. If you can not get uncored rope, then cut cored rope into lengths and snake the nylon core out of it. It is not impossible to do as long as the lengths are under 5-6 feet. Check with Magician's supply
http://rope.ws/ . They often have uncored cotton rope and their prices are great.
I have never tried it, but I heard that hemp rope is more traditional. Maybe I'll make up a batch of hemp rope sometimes soon and try it.
Rick