Have you tried Googling Robinia Twisted Baby - that should bring up sellers.

Or try the RHS website plant finder.

The plant you have illustrated in fact a tree, Robinia 'Lace Lady', reasonably common.

Twisty Baby was its trademark name initially, sometimes sold as Twisty Baby, more often Lace Lady. Does seem to be some confusion regarding eventual height though - Twisty Baby is listed as 6m, Lace Lady as 2.5m, so I do wonder whether they are, in fact, different cultivars. Or they're lying about the eventual height, lol!

Thanks again to everyone for helping getting the mystery solved.