Seeds - I try to buy a fair amount from good vendors. W/o them, the entire hobby will suffer.
Seed trays are small plastic cups in a tray, convenience store carrier, cardboard box with holes, or the like. I'm a fan of starting plants in bigger containers, just to avoid fiddling with transplanting, rapid dehydration in the Aridzona climate, and to avoid roots getting entangled in the drain holes of smaller tray containers.
Plastic pots from the local nursery's discard pile. (Watch for black widows!) Got several 15+ gal pots ready to go.
This year, I'm going 100% Wallybag. At 6+ gal, they're much bigger than the faux-5-gallon pots. More importantly, they breathe so much better than plastic pots, which is good for evaporative cooling and root gas exchange.
Soil - I dunnow. For 2015 I tried ~70% composted mulch and that that didn't work out so well.
I'll re-use it, but at a greatly reduced percentage. Will use much more peat, moo poo, and the occasional bag of quality potting soil whenever I stumble on a sale or torn-bag discount.
For ferts and the like, I try to buy from the nursery on the corner if their prices aren't vastly higher than at the evil Big Box stores. Gonna try fish soup as a primary mid-season fertilizer.
