daltons business on line. costs £10 to join but gives you subscription for a year. Great for finding all types of business plus empty retail units. Go into estate agents and see if they have a commercial side. If you see an empty shop or what looks like empty space above a shop go in the people next door and enquire who owns the building. Good luck.
get your yellow pages out, ring all the estate agents in your chosen area, ask to go on the mailing list, they will ask what you want and try and match up to what you want, but you also get sent shops out of your criteria.
drive around, and look for empty shops or shop for sale/to let.
Google "commercial letting agents sussex" (or something similar) and you should get some results. Also check to see if your local council has a website for commercial premises - I know we have one in Southampton.