You can now purchase Paul Chek endorsed, burst-resistant Swiss balls from the BurnSports web site. Just click on any of the product links below and you will be redirected to chekinstitute.com.

DuraBall Pro Swiss Balls
45cm ball If your height is less than 5' 2"		55cm ball If your height is 5'2" to 5'8"		65cm ball If your height is 5'9" to 6'2"		75cm ball If your height is 6'3" to 6'9"

Sizing a Swiss Ball for using as a chair
Use one size larger than you would need for exercise, and inflate until your thighs are parallel to the floor. The ball will be soft and more comfortable to sit on.