I mix my own. I use up a large trash barrel each year. It works great for me!
Start with a sifter. I made one 2'x2' of 2"x4" boards covered on the bottom with 1/4" hardware cloth (wire mesh).
My formula is:
4 parts finished compost
2 parts peat moss
1 part vermiculite
1/2 part perlite
I have a large plastic bin that fits under the sifter. I add scoops of each item into the sifter and rub it around (wearing heavy leather gloves). When it is mostly gone and just unsiftable lumps left, I add the sifted material to the trash barrel and the unsifted material to a small bucket
After a 2nd load into the barrel, I stir it all around
Repeat, repeat, repeat...
When I'm done, I have a whole barrel of sifted starter soil and 1 bucket of peat clumps. I pound that as best I can with a 4"x4"x4' post and toss it into the compost bin.
I re-use planting 6-pack cels, but I soak them in a mild bleach solution in the basement laundry tub and rinse them 3 times. I have great germination, so no problem there.
I also cut one one cell out of a 11"x22" flat for ease of watering. There is always SOME plant I can get by with having 5 of, LOL!
So, this week's project is making more starter soil.