Actually if you get a cheap on/off t-stat a lamp dimmer on the circuit is a good idea anyway since when those t-stats fail they usually run wide open. You can get them at Lowes or Home Depot for about $12.
If you don't have a credit or debit card but want to buy stuff from amazon.com you can use cash to buy one of their gift cards at any grocery store, then pay online with the gift card.