picked this up at lowes today. http://i82.photobucket.com/albums/j258/god_the_ninja/topviewsanpedro.jpg http://i82.photobucket.com/albums/j258/god_the_ninja/sanpedro.jpg its about eight inches. darkish green. 8 ribs. short spines. 4 inches thick it fits the description to me.
Not sure what species, but (unless that placing was just for the photo!) it'll need a much brighter position, preferably with full sun.
yeah. I put it there just for the picture. It sits on my southeast facing window. It has plenty of sunlight.
lowe's and home depot are 'big box' stores. mainly hardware stuff and various building supplies. they also have appliances and tubs/sinks for sale. the garden sections are quite nice (especially in the larger stores). they have everything from the mundane pots to soil, seed/bulbs/plants to wind chimes and other decorative doo-dads to huge fountains for the garden as well.