Hi iamdog,
You can use eBay pulse to drill down to Electronics subcategories and see what accessories are most popular:
Link hidden: Login to viewAmazon also keep a list of best selling electronics accessories:
Link hidden: Login to viewHopefully that'll give you some ideas, but remember that a lot of other sellers will be using these resources too.
Other ways to find hot selling accessories would be just to read store catalogs or contact some electronics suppliers and ask them what their best sellers are.