Tractor Supply has bags of 40/80 grit black beauty for $8. IIRC they are 50lb bags.
I use a paint mask with organic and particle filters (its about $30 at any big box store) made by 3M.
You are not getting into a fresh air rig for less than $500-$1000.
If you can score on old SCUBA mask or EAB mask they would be the next best thing.
