I'm currently using a two wheel bike trailer to move my timbers which is frankly a little low, but otherwise more than adequate. Welding / brazing a simple frame out of scrapped bikes to make a cart which used a pair of 26" MTB wheels would be child's play if you have any experience with metal working. If you want some inspiration into the types of things that you can build up from scrap bikes, check out Atomic Zombie. By comparison to his constructions a cart should be a day's work if you've already got the bikes!