For my sand I bought a children's playset of sand at a craft store (Michael's) which came in 4 or 5 tubes of sand which I've been using for about 10 years and still have about 75% left. Paid a few bucks for it. The Ocean if you live nearby, same stuff there, free, if you're lucky enough to live near one. (didn't need me to tell you that I'm sure)
Later on I found a dollar store around here that sells decorative sand in tubes for...some fancy purpose or the other, but it's nice because it has bits of shells broken into it and that adds a really cool effect if you paint it up like moss or undergrowth because the little shells look like leaves kind of. Anyway it's only $1 and same as above I'll probably never run out of that one tube.
For rocks I pick them up from nearby and at the folks house or at the train tracks behind my house, infinite supply of various rocks. Lots down by the river as well. I go out in the early morning and just get what I need, wash em at home in the sink under hot water and scour them.
For static grass you can get those peel off tufts from the CMON store, always a crowd pleaser, or buy a big bottle of the stuff from your local model railroad store, again if you're lucky like me and live near one. Those places are a goldmine for basing material of every kind.
What else, cork, you can get that from office supply stores in rolls or drink wine and save the cork. Corkboards at office supply stores are a good call, just break off the cork part, lifetime supply right there. Again I'm lucky and live near a cork tree lol. I walk by it in the morning and if an animal or bird has chipped off some of the bark I pick it up and take it home. Sometimes a big animal (raccoon? dog?) will break off a massive chunk then I'm really set for cork.
Weathering can be done with a box of cheap old pastels sanded down (wear a mask, imperative, wear it!) and saved as weathering pigment. Buy like, 24 sticks of various colors for 5 bucks and make scads of bottles of weathering pigment that way. Hrm well that's about all I have time for, just some thoughts.
Oh yeah, last thing, Birch Trees (at least the birch tree near me) drop seed pods that fall along with little seedlings that are a great source of miniature leaves. A handful and you're set for leaves forever pretty much. Pain to sort the dirt and sticks from the little leaves but way worth it. Paint them in various tones to match the scene.