Most overdrives add a bit of sustain, because overdrive and other forms of distortion all compress the signal a bit. There are dozens of good overdrives, at prices from $40 USD to $400 USD, so you'll want to do a search on that.
I would start by looking at the sansamp. It can be used in the effects loop or as a stomp box. I can get some pretty nice sustain out of little more than my SA and a mackie power amp.
compressors, by their nature, add sustain... But you will get the effect you are looking for with a tube amp, I think. A bit of grind and a more mellow attack/release is what a tube power amp is all about.
i use a Pigtronix Attack Sustain pedal. Awesome pedal! Versatile and fun to use. FWIW, sustain pedals tend to get noisy fast, but this one isn't so bad. Use this with a Boss NS-2 or ISP Decimator and ur golden! It also offers a harmonic overdrive, which blends in a layer of OD to your signal. You can also get light compression, stuttering fx, etc.. try one out
Compressor w/ adjustable sustain (rifle through Bongo's comp megathread). I am a Big Muff person, so you can get sustain for days on most models...
Fitting a Badass bridge or similar can increase sustain, and if you wanna get infinite sustain check out an Ebow or fit a Sustainiac!