I researched and wrote and article on left handed basses a few years ago. Given that only about 10% of the population are lefties, the number of lefty basses available probably exceeds that - this is especially true at the entry and low price end of the market.
Warwick, for instance, offer nearly all of their basses in leftie and Musicman supply their most popular models in leftie. Of course Gibson offer nothing and Fender - possibly the iconic bass brand - only offer one Precision and two Jazz models. The real pain, is that the choice is generally limited and special editions are rarely issued in left hand form.
From memory, when I did the piece, about 17% of basses offered in the UK were left handed (bearing in mind that Warwick offered almost 100% and Fender around 5% of their ranges)
In the past I have gone down the Warmoth route to build custom lefties although I've now gone full circle and back to an 'old skool' American Precision.
Good luck with the research - I'll have a Musicman Stingray HH with Piezo in Rebel Yellow with an ebony fretboard and black hardware when you persuade them all to offer some decent specials!!