You can try asking people if they have free desktop computer they do not need anymore, or volunteer/work in exchange for a free computer. You can also try looking on Craigslist, Ebay, and Amazon Marketplace for cheap computers.
I got my first used Ubuntu Linux based desktop by volunteering 24 hours.
Free Geek is a non-profit organization which gives a way free "used" computers pre-installed with Ubuntu Linux, but you have to volunteer at FreeGeek for 24 hours of volunteering, and they are only in a few cities in the US and Canada according to
http://en.wikipedia.org/wiki/Free_Geek
In addition to Portland, a number of other cities have started their own Free Geek organizations.[5]
Portland, OR ("the Mothership")
Fayetteville, AR
Central FL
Chicago, IL
Columbus, OH
South Bend, IN (Michiana)
Vancouver, BC (Canada)
Seattle, WA
Murfreesboro, TN (Born Again—no longer recycling)
Minneapolis-Saint Paul, MN (Twin Cities)
Toronto, ON (Canada)
Providence, RI
Ferndale, MI (Greater Detroit area)
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This is how I got my first Ubuntu Linux based desktop by volunteering 24 hours at FreeGeek doing varies tasks like cleaning, fixing computers, taking apart computers, house work.