I used to use Aspen, and I LOVED it. I have used Cypress in the past, but it attracted bugs because it was humid, had shards and sharp pieces, and I had to mist it a lot so it wouldn't dry out (I used heat lamps at the time).
You can use the Cypress from Home Depot, I'm sure it would be just fine. You won't need to do anything to it just because it's not from the pet store.
Cypress does NOT increase the chance of RI. IMO, it prevents it from happening because it keeps the humidity level right where you want it so it is not too low (which can cause cracking in the lungs which makes it easier for your snake to get RI) and not too high (which can cause scale rot). If you mist too much to where the Cypress it damp or wet, your snake can get scale rot. So make sure you don't mist it too much, only mist it to where it keeps your humidity levels in the correct spot.