Don, if you have a CO2 fire extinguisher and a pillow case or athletic sock you can make it yourself in a blink. Tape the pillow case over the extinguisher's nozzle, open the extinguisher for a few seconds and you'll have dry ice in the pillow case or sock when you remove it. The pillow case/sock is just there to let the gas escape and leave the solid behind. But it will be much cheaper to purchase it already made.
Supermarkets sell it here out of what looks like a chest type freezer serviced by some dealer downtown or in Ft. Worth. Last I looked you could get a block or you could buy a sack of pieces at $1.50 per pound. El Rancho, Tom Thumb, Kroger, Fiesta Mart, Wal-Mart, Costco, etc. I don't believe as much is bought around here as in more remote places, however. When I grew up it was very common to get it at the locker plant or the ice house and people living out on ranches would get some every week. Especially during hunting season. The locker plant supplied the schools with what they needed (field trips, science classes, halloween carnival, things like that). They had metal coolers and they were always using it for something. I'm certain the ice houses here (and probably where you live) sell it in bulk.
