If you put Stihl 08S in the eBay search window, you might not find any results. However, if you put "Stihl 085," you might get lucky. Many people read the old 08S label incorrectly - due to the age of the saw, it may really be difficult to read and so you just assume it is an 085. I have owned an 08s now for many years and I can tell you that this is a very powerful chainsaw. I use to cut oak stumps that are brought to the surface of bog land that I own, believe me when I tell you, the oak is very hard and it takes a very good saw to deal with it. This is where my 08 super comes in, this saw has no difficulty at all dealing with it. I use a 36" guide bar and chain without any hassle.
Here is a little history about the saw for you. The Stihl model 08 was first introduced in 1963. The 08S (or 08 Super) was introduced in 1965 and the engine was increased in 1969 (to 56 cubic centimeters from 48). There is also an electrical version, the 08SE.
At 18.5 pounds, the 08S is not a light chain saw, but it was very popular in the 1960s. This was before Stihl opened their Virginia plant in the United States and their popularity exploded.
You still can find the 08 Stihl Super chain saw online, but it is getting more and more rare. There have been 3 sold on eBay recently - none in great shape - and they have gone for between $100 and $150.
This is the kind of saw that is probably more valuable as a collectible than a truly working saw, but the choice of course is yours.