Yea, you CAN use pumic stone. But....what is the condition of the local source for pumice stone in your area? How much dust is in/on it? Was it sprayed with something? Do you have to clean/condition it before you use it in your tank? Does "the guy" at the gravel pit where you buy it from know any of this stuff? Do you?
How much cheaper is your local source of pumice stone than Pond Matrix? Pond Matrix is the same as Seachem Matrix, made by the same company, but packaged and sold in bulk. Look at that if you want to save a few bucks.
The risk involved with a local pumice stone source is not worth the few bucks you may/may not save. But, your mileage may vary.