This is a two part answer.
The easy part: you can get the sounds from any any old hardware or software synth module. There are a LOT of old rackmount hardware synths out there for pretty cheap ($50 and up). Even a cheesy beginner's keyboard will get you started, as long as there is a MIDI input.
The hard part: On the bass side (I'm assuming you have a bass that you want to use to trigger these sounds) you have three options.
- The SONUUS plug mentioned before - it looks cheap, quick and dirty but would be a low cost way to find out if you're serious about doing this;
- Adding a pickup, most likely a Roland GK-3B. These are US$200 and can be installed by anyone with some skills (hey, I've done it!).
- Permanent installation into your bass. I believe you can get a GK-3B permanent install kit (the guitard version is available). Another option is piezo pickups in your bridge saddles with the appropriate electronics to convert to MIDI. RMC and GraphTech are two places to start looking. This option requires permanent changes to your bass (routing, etc.) and will involve serious cost, from at least $500 up to maybe $1500 (I'm guessing here!).
- If you want to just buy a synth-ready bass, save up your loonies and grab an F-Bass(!).
You're not done with the hard part yet. You need an interface that converts the signal from the MIDI pickup into actual MIDI data. There are two options:
- (More popular) Converter with onboard sounds and MIDI output - these have been around since the 80's. Roland is the leader in this business (their latest is the GR-20), although Terratec offers the Axon AX-100 which apparently tracks better than the Roland gear. You may also find some old Yamaha gear laying around that did this kind of thing (their G50).
- (Less? popular) Converter with MIDI and USB output. These produce no sound at all, but include a USB connection for direct recording to your computer. Look for the Roland GI-20 and the Axon AX-50, both around US$500.
Just for laughs, here's a ten-year-old site you can check out for some ancient MIDI-guitar history:
http://jpsongs.com/troubadortech/mgtr.htm
DISCLAIMER: I've tried some of this stuff over the years, some of it quite long ago when the technology required more patience than I had. I'm still interested in going this route and plan to purchase this year, but I have no direct experience with any of the newer hardware I've mentioned (well, I did play a GR-20 with a Roland-ready Strat recently, but there's only so much you can do at Guitar Center with their demo gear...)
Now, my advice to you:
Pick up a SONUUS plug and a cheap hardware synth (rackmount) in the near term; you can get used to the technology for under Cdn$300.
Once you know you're serious, a Roland GK-3B and either an Axon AX-100 or one of the Roland floor synths (GR-20, GR-30, GR-33) would probably be best for you (since you're gigging).
CAUTION: some of the Roland modeling products, like the V Bass, do NOT output MIDI note data, so you're stuck with the onboard sounds. Broke my heart when I found that out...