The tremulator is as close as you can get to fender tremolo sounds, its great but a bit expensive.
The carl martin surf trem is also excellent, and pretty cheap
Keeley TR-2 is ok at it, but has a few more options
I use an empress tap trem, which has a tube-style setting as well as square wave and a load of other options, but if you just want fender trem sounds, and money isn't really a problem, the tremulator is the way to go