you generally get more sustain by adjusting the threhsold and release times on a compressor....
Compressor stompboxes abound, varying in price from under £50 to a few hundred. Generally, you get what you pay for. For the cheaper units, you will probably find it very hard to get much sustain without the horrible artefacts associated with over compression.
Strymon do very good pedals, and have an opto-comp they are in the expensive range (~ £200)
The Keeley 4-knob compressor is in a similar price range, but allows direct control over sustain...
I've heard good things about the Dyna-comp which is a bit cheaper.
As far as a dedicated sustain pedal, EHX's 'Freeze' is the only one I've heard of...it's a different kind of thing, no idea of it's merits/pitfalls...