I got mine at TSC, the interlocking stall kits. But they aren't cheap. I see ads on CL sometimes for used mats, so you may want to look there. I reccomend the interlocking no matter which way you go, because if the mats don't fit the stall dimensions perfectly they will shift and it's hard to pick in between mats.
It would be great if you could put gravel underneath, and level it before you put the mats for better drainage. However, we have dirt under ours (more like clay) that was leveled for the barn (not perfectly level) with the interlocking mats on top we are just now (3 years later) starting to have a problem from them urinating in the same spot. However, The one bad spot i just peeled up the mats, got rid of the nasty clay, and filled the hole with sand. It is like perfect now lol.
I would like to put down river rock, level it, and then place the mats back on top. I figure the river rock will be great for drainage since it doesn't pack tightly. but idk, haven't tried it yet lol.
If you plan to get an economy sized manure spreader like the Newer Spreader you are going to have to go with shavings or pelleted bedding, straw won't go through the spreader. Oh, and I know where you can get a truck load of shavings for $30. =).