LEGO doesn't make tracks that can conduct electricity anymore, so to get more you have three options:
- 3rd party manufacturers - I know there are companies making curved track for larger curves than LEGO's own, I can't rule out that there's also companies making other types of track, but there's also copyright for them to think about
- Used parts - curved track is cheap, straight tracks are expensive (most train sets came with a lot of curved track to make a circle, only some with a few straight tracks)
- Make you own - as Ivan's answer details
Another solution is to buy some PF/PU trains motors etc., and rebuild the trains you have to use that, and drive them on plastic tracks. I guess you can find guides (for at least some models) online.