12" is pretty small, particularly for something like an L48.... people say they tend to get clogged quickly unless you're in very dry, sandy soil. A BT1100 easily handles an 18" or 24", which are more productive and handy for general use. But if you really need a 12" bucket for some specific project and have the bucks, maybe a 12" BT1100 replacement bucket would still be available from Kubota - have you checked with an outfit like Messick's? A good dealership like Messick's should at least be able to give you the dimensions and indicate any cross-compatibility with other Kubota or generic backhoes. There were other BH's of similar size made by Kubota; they may have used the same line of buckets.
Also, there are small outfits making attachments for skid steers and backhoes who might have or would fix up something that would fit. You'd have to ask around.
Finally, don't rule out ebay, craig's list, etc.