For my machine I ordered the bushings from the dealer (wallace) and I swapped them out, I only swapped out the lower bushings on the boom as that was where the wear was.
Should not a big deal, if you can't find at a dealer, with measurements you should be able to get from most machine shops.
Contact an old time dealer, even a different brand, they should be able to help you. This sort of stuff is done every day.
Good luck,
Joel