It really depends on which part is leaking oil.
Are you able to look under the car and see what part is leaking?
If so, I can further help you narrow down your leak.
Your oil pan is under the center of the engine, if you hit that on something, it could create an oil leak. It will most likely require replacement. The pan itself will cost about $100, and a shop will charge you a couple hours of labor at whatever labor rate to replace it.
I hope this helps!
Aftermarket should make one for it, just call around to Carquest, Napa, Kragen, or Autozone.
Well, a leak is a result of something not sealing, not the weight of fluid you are using.
At almost 200K miles, it is normal to have some leaks, and they are just something you need to take care of.
The only way to tell is to get under the car, and look to see where the oil is coming from. Look to the highest point that the oil is leaking, and that is your leak.