There are probably a few names but I've often heard them referred to as dress nuts. A search on Digikey using that term shows plenty of fairly similar nuts although they don't appear to have any with the slot at the top:
http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&KeyWords=dress+nut
I'm not sure if specialised tools are available but a pair of circlip pliers from your local hardware store might do the job, they are available in a wide variety of styles and sizes so take the switch along with you to find one that suits. Most have round prongs which you could remove to make things easier.
