I have a collection of resistance bands with handles and bars from www.LifelineUSA.com
ranging in resistance from 40 to 100 pounds. In addition I have a three band with handles set from Walmart which cost me $15.00 USD some years ago. When I travel I find that set still adequate to provide me with an excellent workout. To increase the resistance of a band simply shorten the length of your grip. The resistance of a band is a function of the force needed to double it in length.
Final note, Lifeline USA bands are sold at Sports Authority if you want to test drive bands.