Sb-700, or look for Sb-600 and sb-800 for budget options.
You can get an SB-24 or SB-25 fairly cheaply off eBay. No good?
not compatible with TTL on Nikon DSLRs. You cna fire them manually but there is no point, the SB-600/700/800/900 were design from the ground up for Nikon DSLRs.
What is TTL??
Take a look at the youngnuo range especially the 568, a touch over £100.