I have used SP's from the store, the organically grown ones would most likely sprout quicker for you. White potatoes are treated to slow down sprouting and extend their shelf life so I'm guessing that they do the same to SPs. Organic ones should be treatment free but will still take some time to sprout. Next fall save a few SPs from your harvest and they'll be full of "slips" by mid-winter.

btw, there are mail order sources for SP slips if you're in a hurry.