Changing the front forks 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.