I have the large Kissing Crane brown mule (it's available in three sizes, and can be had for under $10. At that price, you can get several and not worry if one gets lost or broken). It has strong backsprings. It should be noted that, at least in my experience, the steel (or heat treat) in the brown mules I have handled is not as good as Kissing Crane's standard slipjoint patterns. Of course, their regular slipjoints hold an edge better than any other carbon steel slipjoint that I have (Case Cr-V, Eye Brand, Boker).
Another good brand of large carbon-steel sodbusters is Eye Brand. They cost more than the Kissing Cranes, but are still a good deal. I would rate the edgeholding about equal between my Kissing Crane brown mule and my Eye Brand sodbuster, with perhaps a slight nod to Eye Brand. The blades are better executed in the Eye Brand sodbusters, nice flat grind with a fine edge.
If you look around you might be able to find a Case sodbuster with Cr-V blade, I don't know if they still make them but I used to see them around.
For the brown mule and the Eye Brand sodbuster, try knivesplus.com. A lot of the online dealers will carry one or both brands. I have also seen quite a few of the brown mules on ebay. Just remember that they come in three sizes. The Eye Brand sodbusters also come in small and large sizes.