The proper seat can boost immersion
What you're sitting on as you use your simulation presents you with another opportunity to enhance the experience. If you use a fairly comfortable chair you're familiar with, say an office chair, it likely will fade from your awareness, neither adding nor detracting from your experience. If you are planning on building an immersive environment for your sim, having a seat that looks and feels like a flight rated seat can help. Seats used by airline crew do not look like office chairs, and fighter jet seats are way different.
Check out some examples
There are several sites by hobbyists that will give you some indication of what can be gained in this area. Take a look at Matt Wietlispach's site to see what effect adding a real ejection seat can have. You don't actually have to buy a real flight rated seat. Making a seat is a quite doable project and one that you might seriously consider. Martin Schmitt has an incredible F-16 simulator amply illustrated on an equally impressive website.
If you want to research ejections seats by all means visit The Ejection Site.
Fabric
If you're building a replica seat and need the real-deal aircraft upholstery fabric, Spectra Interior Products, Inc. supplies a substantial portion of the airline industry with their material. The stuff's not cheap. It runs about $50 per linear yard if you're buying in small amounts (less than half a roll).
Spectra Interior Products, Inc., 1949 Stonewood Drive, Winston-Salem, NC 27103 www.spectra-ip.com, (336) 794-3030
Add tactile feedback for even more immersion
Beyond the look of the seat, you can also go for a bit of "feel". Thunderseat was a commercial product coupling a seat with a woofer that can be tied to your computer's sound card. It allowed you to feel the engine noise as you fly. Thunderseat is no longer available, but there are alternatives such as the ButtKicker from Guitammer, and the Tactile Feedback System V2.0 (aka TFS2) from Intelligent Vibration
There is certainly no reason you can't add this embellishment yourself, using inexpensive automobile sound system components. You might get a good price from the automotive department of your local discount store. Other possible sources are MCM Electronics and Parts Express Electronics . They carry a wide variety of consumer electronics items. Occasionally they have amplifier modules that look like they would be just perfect for this. If you're planning to build a nifty seat for your sim, leave room for a bass shaker or two under the seat cushion or behind the backrest.
If you want to customize your sim's sounds, check out Historic Aviation for some CDs of aircraft sound effects, and Avsim.com for some sound downloads.
G-seats...
Another clever thing you can do with your seat is to add a set of inflatable cushions that respond to the g-forces of your flight. This was used to good effect in the now defunct Mission to Mars ride at Disneyland. It's still used in military simulators such as the units produced by Link Simulations and Training. As the pilot experiences simulated acceleration the cushions deflate a bit causing the pilot to sink into the seat. G-forces cause the pilot to shift in the seat, altering his or her viewpoint within the cockpit as well as altering the perspective of the outside environment relative to the canopy structure. This shift in perspective is a very important cue to the pilot about the vehicle's performance and flight dynamics. Adding simulator controlled cushions to make a G-seat allows these cues to be incorporated into your sim.
And arm loading
And now for a final bit of embellishment… This is not specifically a function of the seat, but in a simulation the seat provides a reasonable base for the mechanism used. When a fighter pilot is making extreme maneuvers, g-forces are yanking his arms about, to the obvious detriment of his ability to control his vehicle. Some military simulators have emulated this effect by mechanically pulling or pushing on the pilot's wrist. If you are building a seat with pneumatic cushions this feature may be a straightforward extension, utilizing a pair of pneumatic cylinders to tug or push at your arms.