Like others mentioned, bikes are always looking for homes. Ask relatives (sis inherited my old hybrid that I swore I was keeping--until the LBS determined her bike was unfixable and then it was hers for the asking!) Garage sales, newspaper ads (I know old-fashioned, but I think people still do that instead of Craigs List!) There is also a reconditioned bike shop in my town--they donate to charity or sell to raise money for more bikes.
Of course, you want a bike that fits properly--but anything is a start and will help you to determine where to go from there.
"Well-behaved women seldom make history." --Laurel Thatcher Ulrich
'09 Trek WSD 2.1 with a Brooks B-68 saddle
'11 Trek WSD Madone 5.2 with Brooks B-17