A dual-density foam roller is only 20 bucks or so, and they last a good bit longer than the normal ones. Typically they are blue, and have a center cavity made up of a separate foam piece. They are also available in travel size, which basically is the full size cut in half. I'd recommend sucking it up and shelling out the cash.
alternatively, i've used coke/water 1 liter bottles, still full and unopened. they hurt a good bit, but were effective when i was out of the country and couldn't get my roller. Make sure they're unopened, otherwise the cap can come flying off, and that makes a giant mess.