Fresh horseradish is difficult to source. Try a good farmers' market or specialist supplier, or look online. Keep fresh horseradish in a paper bag in the fridge for up to one week, or cut it into ...
Buy firm, fresh horseradish roots in whatever quantity you wish. Since the sauce will freeze well, it is more efficient to process a larger amount at once if you will be using a lot in about a year.
Homemade horseradish sauce from scratch is a revelation. Not only is it ridiculously easy to make, but fresh grated horseradish packs a far greater punch than any ready-made bottled variety.