You can get sound machines for $20 at Bed Bath and Beyone ($16 with coupon) - Homedics brand. Ours is still going strong after 2+ years and works as well as the $$$ one from Brookstone that we got with DS1. You can also download a loopable audio file from www.simplynoise.com - they have white, pink and brown noise. Many people also swear by the Marpac white noise machines, so take a look at those too. A fan would work, but I can't stand air blowing in the room, so I have never tried it with my kids (not to mention I don't want them playing with it during the day).
Mama to three boys ('03, '05, '07)