The model is not really important but the frame is. Look for a frame with no front brake, high rise handle bar, swing arm with bicycle type chain tensionners. As to where to buy, i always buy mine in canada so I cannot really help you.
Try to avoid this type of frame :
http://www.pocketbikesunlimited.com/images/IMG_0408.jpg
Depending on your weight, Foot rest will bend and touch the ground, swing arm will come so lose that you'll have major chain problems and rear shock mount on the frame will break.
