Welcome kebrown
If memory serves me correctly, the town of Amherst offers free compost/mulch for residents or for next to nothing. The it the best I can tellyou as I have to make my own or buy it at a garden center and since I need so much for our heavy clay I make my own.

Another option for your patio slab (we are planning on something similar hopefully this summer) is to put planting beds around the perimeter of your patio. The raised area would help hold the soil back off the patio.

Penny

Amherst sells compost at a great price, and open to anyone, not only residents. It's NOT sewage, so no heavy metals and ok for veggies. I buy it by the truckload - $20 for a full size pickup heaped. Or you get it by the bucket. It's great stuff, however, it is full of little plastic pieces from landscapers who dump all their trash, not just plant material. So you will probably have to spend a fair bit of time pulling all that plastic out.

I also buy shredded wood mulch from a place on Route 33 just west of Corfu. Much more expensive ($60 per truckload) but still lots cheaper than bags, and has no plastic in it. So it's good for mulching the top. You may not want to drive that far. $40 truckload for big wood chips.

Topsoil I get from a local excavator. They all seem to charge about the same. $20-$25 per yard delivered.

Here is a link that might be useful: amherst compost facility