What's wrong with laying the black tube on a hot concrete driveway with a brick on each end? (besides that we're running out of hot days)
Besides Greenspeed you may also try Trisled and Ian at Flying Furniture (as he sells Bacchettas and they use a dual idler system). You can even try making them yourself. Whatever way you go you'll need to buy either bearings or the finished product according to the axle or shaft that they fit onto, unless this was damaged too and you're making something different. By this I mean that the Internal Diameter of the bearing needs to be only a smidge greater than the Outside Diameter of whatever this all fits onto, if the ID is a lot greater than the OD then you'll need to pack the shaft, if the ID is smaller than the OD then you bought the wrong product and need to go shopping again.
Edit: These are on the WISIL projects page about making idlers. Do so at your own risk, I have better equipment such as a lathe and milling machine to do this.
http://www.recumbents.com/wisil/Idler/leger_idler.htmhttp://www.recumbents.com/wisil/idler/idler.htmAnd strangely enough I got the solid plastic rod offcuts to make the idlers from a bearing shop, you don't have to mug some kid in the park for his skateboard wheels.