Here's what I recently said to someone who PM'd me this question:
Unfortunately, brands really don't matter. True, bland topsoil is very cheap - around $2 for a 25 pound bag. Unfortunately, topsoil is also very heavy. You can probably see the problem here; there is very little payoff for companies to ship topsoil very far. I used EarthGro brand topsoil, but I wouldn't recommend that you use it. I wouldn't recommend that you use anything in particular, actually. Rather, you need to figure that out. I recent had to switch brands because EarthGro is no longer available where I live. Follow these steps, and you'll be fine.
Go to your local hardware store, such as Lowe's, Home Depot, Menard's, and so on. Avoid specialized gardening shops, as their topsoil is "high quality"... good for plants, bad for tarantulas. Find where their topsoil is, and ensure that it's not kept near the pesticides. If it is, go somewhere else. Read the ingredients listed on the bag (if no ingredients are listed, move on). You want to avoid cedar and pine like the plague, because it will kill a spider. Thinks like forest compost, sandy loam, and fine mulch are just fine. You are wholly entitled to slightly tear open the bag to look at the consistency. Don't get anything that heavily resembles mulch. Inconsistent texture is fine... it's dirt. It should look like dirt. Buy a bag or two, depending on how much you need. On the way home, swing by a pet store and buy a handful of large and small crickets, no less than 10 total. Put some soil in a ventilated container, throw in a slice of carrot or potato, and put the crickets in there. The food is necessary, as they will certainly eat each other if no food is available. Leave them for 48 hours, and see if they die off. 10% death isn't anything to worry about, but anything much beyond 30% should be cause for alarm. Anything past 50% means that you shouldn't use the soil.
If you passed the cricket test, then you're good to go! Note that if you put the topsoil in an enclosure while it's still moist (it's moist when it's fresh out of a bag), it will absolutely mold... no big deal. The mold will very quickly die off as the topsoil dries out, and it won't harm your spider. If you have a species that requires humidity, you still need not worry. Bland topsoil has very few nutrients, so mold uses up the nutrients very quickly and, again, dies off. You just need to clean up any boluses, as that will be a food source to mold.