If you use an alternative brand of horseradish make sure it is pure prepared horseradish and not the mayo-like creamy stuff. You may not have access to Creole mustard, so try a quality spicy brown mustard, but I can't guarantee the results. I urge you to order the 16 oz. Zatarain's Creole Mustard online
. You will be tempted to skip the step of drying your shrimp with paper towels to prevent excess moisture before adding them to the mustard mixture. Don't. If you do, the shrimp dip will be too watery. For best results, make the dip a day ahead and refrigerate in an airtight container overnight. (Added note: To prevent excess moisture in the dip, do not use pre-cut frozen vegetables (onion, bell pepper, or celery), since they will become watery when thawed.