They are all over ebay for reasonable money and you don't need B&S or starrett accuracy. I'd never disagree with Bulgie but, as an alternative, to the v-block approach, you could visit Paragon and get their tubing blocks. If you get all the same cube size (eg 2"), it maintains the tube centerline of the frame regardless of tubing size. v-blocks might not do that depending on size, orientation clamping mechanism etc. Paragon blocks can also be used in a vise/mill for clamping the tube, you can drill them and attach other little doodads to hold braze ons or tons of other stuff. They are really useful. Just my $.02.