you can reuse your old sand. I clean it by putting some in a 10 gal
bucket, stir it up lots so most of the mulm is suspended in water, then pour out the water. You would lose a bit of sand, but it gets rid of a lot of the stuff.
The best place to get sand & gravel in my area is a landscape supply place. I can get a 2 cu. foot bucket for $2. <They have 1/2 a dozen types: play sand, coarse sand (=PFS) ... etc.> They also have an extensive pond supply section where I can get fish safe rocks, plants & ferts.