I had great luck putting an ad on Craigslist. I got four dead wheelchairs donated to me, and was able to make one working wheelchair robot from them with a heap of spare parts left over. I put an ad in the "wanted" section. I asked for electric chairs working or not, and explained what I wanted to do with them. I had several generous people call me. If you have something like craigslist there, it couldn't hurt to try it.
You might also see if there are robotic clubs in your area. Many times they'll trade parts with each other. A bit further out there, but you could also write a letter to local hospitals or nursing homes asking for their no longer useable chairs instead of them getting scrapped.
Get creative. It worked for me, and I met some great people in the process.