I have an older trek 8500 aluminum bike with rigid forks and I would like to change them to be suspension forks. Is this possible? Btw the tire size is 26".
If original, this is an old bike - the Trek 8500 went to shocks before 1998.
The 1996 is listed as having a 1 1/8" head set, so technically it would be a straight forward swap with a fork with the same headset (Most are now tapered). A shock will lift the front of the bike, changing the geometry. A smaller travel shock (under 100mm) might be acceptable and not change handling significantly.
You would need to find a 9mm qr fork with brake bosses for V brakes - (hard to find now unless very cheap model, although possible to find used ones in good condition, theres not certainly of it.) The 1996 had V brakes, but earlier bikes might have had canti's. Upgrading brakes to discs (new front wheel) would be an option if you cannot find a fork with bosses for V brakes (requires a new front wheel).
Overall, unless you have a real affection for the bike or it is in exceptionally good condition, the cost will be prohibitive. Most people (myself included with a similar spec 1996 KHS I now really miss) end up doing the home work and opting to upgrade the entire bike.
Yes, should be fairly simple, most suspension forks are for 26" wheels. You need to measure the diameter of the fork tube, or it may be stamped on the tube itself, 1 1/8" is common. You need to remove it to check.
Also, most new forks use a threadless headset, the stem slides directly onto the fork. But older styles require a threaded stem with internal quill.