You guys will laugh at this, but we bought a bale of Pro-Mix on Amazon and had the sucker delivered.
We could have bought it at Southern States locally for less money but we would have had a terrible struggle getting it into the car and out and back into the shed.
Amazon Prime is rather amazing.
Once that big bale was on the front porch, we put it on a little dolly and trundled it back to the shed.
In the past, I have used regular potting mix for my winter sowing (organic varieties only) mixed with some additional vermiculite and sometimes coconut coir. So using Pro-Mix will be interesting.