where could I acquire plans to build a Kart/Buggy?

If you're building a Kart then you don't really have any kind of suspension to worry about, and most of what you need is a frame to hang the steering/wheels on,and somewhere to put a small engine (almost forget you need a seat too!)

All you usually need to to do is make sure that the frame is strong enough to minimise flexing/twisting (if you've prevented flex/twist it's unlikely to bend in the middle) It's also probably a good idea to have some kind of roll-cage/roll-over bar just in-case you manage to flip it. The TV program scrap heap challenge often has vehicles just like that, and should give you the gist of what you need. Unless you are really pushing the limits of performance, you shouldn't go far wrong.

There's a book by Ron Champion called "Build Your Own Sports Car: On a Budget" (ISBN 1-84425-391-0) It should contain any mathematics/ theory that you need to calculate the loads on the frame, and the dimensions of the frame to cope with them. If you really want to get into building/racing vehicles then you might want to check out some of the technical books Colin Chapman (of Lotus) wrote.

You could always check out the chassis used in kit cars. Many kit cars are a frame, with a body bolted on to it,and as the chassis is such a vital part of a car, the manufactures of the kits are often keen to show of the "skeleton" of their vehicles. If you ask nicely you may be able to get a plan, for minimal cost.

You should be able to purchase schematics from most kart/buggy manufacturers .

google it! bound to be some you can dowload