Im not really sure aftermarket glidecam is really the right words.. Thats like saying Brooks are aftermarket Nikes or Surfaces are aftermarket Armadas.
I got the Wondlan Ares, its not much if at all cheaper than the glidecam but it looks pretty legit. I dont have a video camera of it right now to do any of my own testing but heres some videos.
阿飛衝浪MV 幕後拍攝花絮 from Mstudio (Michael 0302) on Vimeo.
This is the video that convinced me to get the Wondlan.. Its fairly long though, just skip to the running section if you want..
Review 3 Different Stabilizers for DSLR Video from Anthony Martin on Vimeo.
Heres the only place that you can buy it I believe.
http://cgi.ebay.com/WONDLAN-ARES-stabilizer-Steadicam-DV-Camcorder-5D-7D-/220692923479?pt=AU_Video_Cam_Accessories&hash=item336252c457
Though I bought mine used, but Im fairly sure you can find them for around $275 new on ebay.