I haven't a clue about sizes of baby baths (although wish I'd looked into it with my long girl!) but have you tried bathing DS in the laundry sink? My MIL did this with DH and also bathes DD in the laundry sink when we visit her. The beauty of it is that it is a decent height in comparison to a normal bath...
Another option is to purchase a big container from the hardware store or something like a clam from a toy shop... I've seen both done...

HTH

MG