If you know their user ID or store name, you can use the advanced search option (click the small grey 'Advanced' link next to the search bar), just select 'by seller' or 'find stores' depending on which name you use to run the search.
If you've bought from them before, you should be able to locate the item in your purchase history or feedback profile and find them that way.
If all else fails, you'd probably have to run a normal search on an item you know for sure they sell, refining the results as much as possible and locating them that way.
