I know the best mix involves perlite, worm castings, builders sand, pre-mix of (12-6-6?), etc. But is there a packaged soil that is like that already?
You won't find worm castings or soil with worm castings mixed in at WalMart or Home Depot. And while you will find soil in those places with ferts already mixed in you do not want that.
This time of year it might be hard to find what you need, but look for a pure organic potting soil. It might be 85% or so organic matter and 15% or so sand, that's fine.
If you want worm castings (and as far as I am concerned you definitely do) get them off eBay, try kdfarms, or try a local nursery, real garden center, or hydro shop. Then mix it yourself. Or, buy a premium soil like FoxFarm that has worm castings already in it but for a considerably higher price than buying separately and mixing your own.
My typical mix is:
40% organic soil
30% sphagnum moss
20% worm castings
I might adjust that if the organic soil has a lot of peat moss already in it. Sometimes I throw some sand in as well if the soil didn't have it pre-mixed.