Online shopping from a great selection of bike seat clamps in the Outdoor Recreation store on Amazon.com.

The easiest do-it-yourself seat-height adjustment is done on a trainer or indoors in a doorway and requires a friend or spouse to help. Put on your cycling shorts and shoes, mount your bike in the trainer or place your bike in the doorway, get on and hold onto the doorjamb to support yourself. Have your helper stand behind. Adjust the seat height by loosening the clamp where the seat post goes into the seat tube. Move the seat to its new position, then tighten the clamp. (If you're a big guy riding a smallish bike, make sure you have three inches of seat post inside the seat tube!) This is a quick-release seat post clamp.