It depends on how much you're willing to spend. You can always line a shoe box and use it or even use a milk carton. Wood molds have to be lined which is why a lot of people prefer silicone molds. Other people prefer acrylic which you also don't have to line although you may want to spray it with a food grade silicone or swab it with cyclomethicone.
As Soapourri mentioned there are molds for sale on eBay. You could also check through the Shopping Recommendations section for info on suppliers. Off the top of my head, some suppliers are Brambleberry, soap-making-resource.com and Wholesale Supplies Plus.