Is there a twin turbo kit for the nissan silvia s13 sr20det? And if there isnt how much do you think it would cost to have one customed made?

There really is no advantage to a twin turbo setup for any inline engine. All I would do to get more boost would be to get a larger turbo, biggest intercooler you can, and a fuel system that can supply enough. The only problem with that is the increased lag, which can be taken care of by smaller intercooler piping or a shot of nitrous.

The only twin-turbo conversion available is from GP Sports (www.gpsports.co.jp) but its for the S15.
Transplantations of RB26DETT engines from Skylines are also possible. I dont know the cost of it but i have photos if you are interested.