I participated in a thread around Passover last year in which many CHs agreed that it's very hard to locate horseradish that's fiery enough. Since then, I've found Saw Mill Site Farm's Frankie's Original Hot Horseradish. It's a local company whose ad I saw in Edible Boston. I called the company and had a wonderful chat with the owner who assured me that his is the real deal. When I mentioned that my mom usually grinds her own horseradish but that we've had trouble getting roots that are potent enough, he said it's because most roots that are available to the general public aren't big enough to be as fiery as the ones he gets.
I haven't opened my jar yet because I don't want it to lose potency before the seder so I can't give a product review for a few weeks. It's $4.69 a jar, so a bit on the pricey side, but I think a small price to pay for supporting a local business who's so friendly. The "where to buy" info on the website isn't entirely accurate but it is available at the Whole Foods on River St. in Cambridge, Henrietta's Table in Cambridge and at Pemberton Farms in Cambridge.
For more info., visit www.horseradishdirect.com .