Only 2,600 jumps for me, so Robbie has me beat. I've used a reserve parachute (the second parachute us jumpers carry) 4 times in all my years of jumping (one of them is a particularly good story to tell over a beer at Airventure!). And the most important thing, as Robbie says, is to learn how to use it. If you go to any skydiving center (
http://www.uspa.org/dropzonelocator) they will have a "hanging harness" that suspends you in a parachute harness a couple of feet off the floor. You can practice pulling an actual ripcord handle and get used to being suspended in a harness. They can teach you about steering, etc. Take the opportunity to hang around and watch the jumpers do their gear checks before getting on the plane, and watch them land. If you really get the bug you can make a tandem skydive where you'll get to actually steer the parachute while harnessed to an experienced jumper.
As for the type of parachute for a Sonex/Waiex, a seat cushion type may not feasible simply because of the lack of head room, unless you are vertically challenged and you remove the seat cushion first. A seat-back type might work best, but even then you will likely need to remove the seat back cushion so you are not jammed forward to the instrument panel.
I've never flow a Sonex with the canopy ripped off, but that's what you'll be doing if you have to get out with a parachute. It might just fly like a brick, I don't know. If there is any spin to your descent it will be very disorienting and will likely impede your exit. You won't care about making any kind of a pretty exit; just get the heck out any way you can, clear the aircraft and pull the ripcord. We used to tell observers who went in the skydiving aircraft (everyone on board a jump plane has to wear a parachute, even the pilot) to just roll out the door, count to 3, and pull the ripcord, no matter your body position. Most people will probably count to 3 in less than 1 second! The point being they would be in a panic situation and not have any training or useful train of thought, so the simpler the instructions the better chance for success.
And as Robbie said, you should only buy a used chute if you can verify its condition. That means you have it inspected and repacked by a parachute rigger as a condition of purchase. Every chute will have a seal on the ripcord pin, with a red thread and a pressed lead seal. The lead seal has the rigger's unique letters on it, and the rigging card will be signed and dated with the same letters. If it was a recent repack you can try contacting the rigger and ask them about the chute. All parachutes have to be repacked every 6 months, so if the last repack date on the card is more then 6 months ago then it has to be repacked anyway. If it's within 6 months then it SHOULD have been thoroughly inspected by the rigger at that time.
And Barnstormer's is another place to look.