iv used 2 old bikes i got off gumtree but any two old bike should give you the right parts but make sure the parts you want off the donor bikes are good (bearings are easy to replace and cheap so don't worry about them.
the head tubes iv just used 2 off the donor bikes and cut them down so they are the same size i already made sure the diameter was the same but then there may only be one size i don't know lol. there is a free how to bit on the atomic zombie website on how to cut them down to size.