First thing to do is to place an ad in our Swap Meet
After that, start scouring your local scrap yards, craigs list, etc. I assume you've read the Tech Tip on this? It explains what trucks you are looking for (for sale or in junkyards). You might have to buy the whole truck just to get the axle 3rd member you want. The plus side to that is that rusty/wrecked '68 to '72 3/4-ton trucks are probably going to be priced at near scrap value. Buy the whole truck (after you confirm the ratio), take what you need, sell what you can and take the rest to the scrap man. Scrap is up right now so you'd better start looking soon.
Look for your local Rock Crawler group. Someone there might want to trade you for your lower ratio. That's how I got mine.