mix your own, I was using MG potting soil and mixing a lot of perlite in it, which worked and MG is pretty good based on what I saw, but the Perlite I have noticed really just takes away from the room the plant can use. and it is expensive to buy enough bags of soil usually, unless you an get it from some place in bulk..
but I just mixed mine and after trying a few different combos I think I found one that is really great, I will change it a little when I go to a 5 gallon bucket, but that is just to add some worms and worm casings (I have a place near by where it is an overflow area for a river, and the soil there is almost all worm casings and there are more worms than I have ever seen anywhere else.. so I am going to take a bunch when it gets a little warmer haha)
but here is the mix that I just mixed up, and it has great drainage so far, it is heavier than some, but once the roots get established its great because it drains really well. and the goal for me was to cut down cost while still having a good soil, so I had used the compost soil from a leaf pile I had for.. well ever really, it's just been sitting in the woods beyond the yard, but you could use any compost, and I had a mix with some garden soil, but I decided to not go with it for now for the young plants so they can get some really good roots going.. and of course you can tailor it to what you need and what you have around you but here it is
5% wood ash (from a wood stove, it has great nutrients/minerals in there, and some potassium I think I read on here, but it's apparently really good, instead of sifting it like I did the first time, I kept all the little coals in there too and just smashed them up into tiny pieces, and if you don't have a wood fire, just go outside and burn some branches or something for a while and once it goes to ashes, use that!)
25% Peat Moss (in the 5 gallon buckets, I will take 10% of this and use the worm casings/soil+worms instead)
15% compost from leaf pile sifted through a sieve I made with I think 1/4" holes, maybe 1/8th" holes
10%shredded pine mulch (I wanted to use this because I used it last time, and I figured it couldn't hurt, instead of using all chipped pine bark)
30% Pine bark chips/mulch (you can get it at any nursery or I got mine at Agway, just ask for the really tiny chips, of pine bark, they are almost like thicker shavings kind of
15% Vermiculite (I switched from perlite since the perlite didn't hold water well, and it mainly just floated or came to the top of the soil and let the soil wash through it if that makes sense. but the coarse ground vermiculite looks promising and is very airy
I added just 1-2 tablespoons per 2 lbs of peat moss, I didn't want to make it neutral pH but I actually have to check it now to see what the pH actually is, and I just made sure that the peat moss and vermiculite were damp before mixing them because if they are dry they take a while for water to soak in, and they can cause problems once everything is potting up sometimes
I also added some bone and blood meal and some potash and just mixed it in, I did about 1/2 the recommended amount for the amount of soil I had, I usually mix them individually in the gallon pots, but since I was potting up seedlings into solo cups, I just mixed it all into the soil really well, and since the bone and blood meal take a while to break down and are slow release, I fertilize them a little with liquid fertilizer and Cal-Mag each watering at low doses. it is working great so far! I actually have to pot up the 2nd group of the seedlings tomorrow, and find some room and which light to put them under lol I need some warmer weather so I can stick them outside
Oh and if you don't want to mix it yourself, Promix BX (with microrizhea (sp?) if you can get it, because the Mykos in mine I think is doing a pretty good job) mixed with some compost/soil mix or something should be pretty good, but really mixing your own is generally much cheaper, you can get at the very least 2-3x as much soil if not a lot more for the money you spend on the mixing stuff rather than just buying soil yourself