Hello and welcome to the forum. We had a discussion about this a while back and I bookmarked it, follow the link below. - Well, time does fly, the discussion is from 2010 so some of the links in the link may not work.

I also receintly posted the following at another forum:

Pioneer Landscape also sells a good product (Sandy Loam) and may be more convenient. Singh is Saturday's only and open just a few hours. (NEC of 101 and Thomas). It's best to mix these products with some native soil. imo, they are not designed to be used alone, although that is not what the sales person will tell you. :-\
Today hubby went and picked up 'Mulch Mix' from Preach Building Supply in Sunnyslope. $14.50 for 1/2 yard.

Good luck with the garden!

Here is a link that might be useful: Bulk Soil in the Valley

Organic soil amendments can be found in the local Phoenix area nurseries: Bakers, A&P Nurseries, Harpers, Root Phoenix, Garden Guy, Summerwinds. Lots of products, some especially formulated for the area.

Ace Hardware, Home Depot, Lowes, Walmart Garden Center, Target Garden Center often carry such as well. Less variety, but more bulk and maybe lower price.