Aside from the 'for sale' section here where one or two have been advertised recently - Have a look on
bemsee in for sale section to see what bikes and bits are kicking about and they also have forums. Its also worth having a flick through Bikesport News mag (website
here), as there are loads of track bikes for sale in there private and dealers. (Eg - '98 Duc 916 with 996 engine, Ohlins shock, recent over haul etc. £4300 ono .... 2000 Mille 1000R, immaculate, MRO eligible, lack of funds forces sale. £4,375 ono - Both private ads)
Have you considered a Mille if you want to track a v-twin?? A lot more bike/performance for the money allegedly. The guy I bought my gixxer 600 track bike off bought a Mille to race on Sound of Thunder and is very impressed with it.