I wrote about my soil mixture that I use for my raised beds here a few years ago. It’s a mix of 1/3 peat moss, 1/3 vermiculite and 1/3 manure or compost.
It’s essentially “Mel’s Mix” from the Square Foot Gardener Mel Bartholomew. Here in Edmonton you can find the large 3.8 cubic foot block of peat moss just about anywhere (Walmart, Canadian Tire, any Garden Centre). I buy my 118 litre bag of vermiculite at Apache Seeds and the manure wherever I can find good selection. Altogether it costs about $52 to make up a large batch of soil which I use to supplement my existing beds or make new ones.
Some of my raised beds don’t really need more volume, they just need the addition of compost or manure. Every fall I dig in leaves and grass cuttings which help to lighten the soil, then in the spring I like to add some composted material as well. This keeps the garden healthy and helps with moisture retention over the warm summer months.