Well, since it's something of a starter bike, there isn't likely much of an aftermarket. Try
Dennis Kirk for general accessories that might work. Owner's manuals are available from a dealer or Ebay seller, and you can do without one if you have a Clymer or Haynes manual and access to your friends at suzuki-bikes.com.
As far as all your mechanical questions... Oil is usually changed at a 3000 mile interval, or every year, whichever comes first. Lube your chain every 300 miles and clean it every few lubes. Warmup time depends largely on it's state of tune - a bike with gummed carbs or one out of synch will take longer.
Specs are available by typing "google.com" in the address bar, typing "suzuki gs500 specs" in the search bar, and clicking the relevant links. Here, I'll do it for you:
http://www.google.com/search?sourcei...ki+gs500+specs
Calm down!