Hello David. Lots of opinions out there, and I'm happy to provide my $0.02 to help with your inquiries.
What do I look for in one?
Look for one that's complete and has the correct parts. Make sure it has a 66mm 090 cylinder and not a 58mm 070 cylinder. The quickest/easiest way to differentiate is looking at the top of the cylinder with the shroud removed (or, you can often tell by pulling the spark plug boot and shining in the gap between the shroud and cylinder with a flashlight). The long skinny piece of spring steel used for the decompression valve runs back-to-front on the 66mm cylinder and right-to-left on the 58mm cylinder.
Look for a six shoe 090 clutch and not a three shoe 070 clutch.
The correct 090 fan cover has 'straight' ribs where an 070 fan cover has curved/concave ribs.
Make sure it has a functioning wind vane governor located inside the flywheel housing and linked to the choke shutter on the carburetor.
Check that both the automatic and manual oiling functions work.
Now, excluding the fan cover, what I've noted above boils down to function not aesthetics. Depending on what your plans are for the saw, making some concessions on parts can save you money.
This varies wildly based on the sub-model, condition, and source. A friend of mine just bought a standard 090 for the seller's asking price of $250, and a never-fueled late model 090AV recently reached bidding on ebay to $3,400 and didn't meet the seller's reserve. Both of those prices are well outside of the norm. If you want to know the high-end of the market search around for completed auctions on ebay and go down from there for a fair in-person sale.
My understanding is that the 090 was manufactured from 1968-1984. Any year would be a good choice. The primary visual difference you'll find based on vintage is that the earlier saws had "Stihl" cast into the shroud and a sticker with the model number on the air filter cover whereas the later ones had metal tags for "Stihl" and the model number. Also, the later saws had "Stihl" cast into the clutch cover and the earlier ones did not.
There are a lot of knock-offs and aftermarket parts out there, so if that's a problem for you, you'll definitely want to do some research and keep a sharp eye when evaluating them.
Where's a good place to find one?
There are a lot of 090s out there, so if you're willing to take time to look and pay a fair price they aren't difficult to get your hands on. They're on ebay a lot, but that's obviously the high end of what you'll have to pay for one. You can always post a want-ad on forums as you have here. Outside of that, keep an eye on Craigslist. You're a little south of the hot zone, but the west coast is a good place to start looking!
What else do I need to know?
Be clear on whether you want to purchase a runner or a potential project. There are a lot of 090s that have been sitting around for many years untouched and need anywhere from a little to a lot of work to get back into running condition.
There are four basic types to look for. There is the standard 090, the gear-reduced 090G, the early 090AV, and the late 090AV. You'll be able to purchase a standard 090 for notably less than a G or late AV. The earlier AV generally brings just a little more than a standard 090.
On the standard 090 there were three types of handlebars. The most popular among buyers/collectors is the seven-bend full wrap. There is also a six-bend full wrap and a normal 'half' wrap.
Most of the 090s I've personally had with good top ends made right around 150 psi. A little lower isn't a huge problem, and given a stock setup saw you normally won't see too much higher.
Good luck with your search, and let us know if you have any questions.
