They are sized for the model series of tractor. So you're looking for ones suitable for the "L" series or, preferably, "Grand L" Kubota models. I think the 3 point setup on the Grand L's remained the same for the 30, 40 and 60 series.
There are a bunch of generic linkage kits on Ebay at the moment that claim to fit several brands of compact tractors, including Kubota. These are quite a bit less costly and somewhat less refined than the genuine Kubota linkages. I don't see any being promoted on Ebay right now that look to be the right size for your L4240. The generic kits may or may not be suitable for your tractor... you might score a bargain or find out you need to send them back. If it were my tractor, I'd keep looking for the correct OEM ones.
Assuming the original linkages got orphaned from the tractor at some point... no chance a previous owner would still have them? And if the tractor came to you from a dealer, the linkages should have come as part of the purchase, I'd think.