Too late for this year, but how do you keep horseradish strong?

I'd agree to getting domestically sourced roots. Just as an FYI, my state, Illinois, is the nation's largest horseradish-producing state. Next, timing is everything. The root is at maximum heat about 30 minutes after grating and then starts to dissipate unless you add vinegar (which, as noted below, is not appropriate for the Marror bracha) to stabilize and maintain the heat level. So buy local and grate just before the seder starts. Doing it earlier in the day or a day before reduces the potency.