You have to measure your wheels, use a calculator and figure out the numbers. Letting someone else tell you spoke lengths could end up with incorrect sizes... Its not hard to do, and there is plenty of info out there. I use spocalc and it works just fine.
I usually round down since spokes will stretch a little, but since spokes ususally come in even # lengths, I round up when it makes sense (like 285.3 would get a 286 spoke most likely).
Just my $0.02
