I've managed to buy the Novation Bass Station VSTi (it's 64bit).
It isn't free... Cost me £26.
http://www.focusrite-estore.com/novation/ You have to slash your wrist and give blood before they let you buy it (name, address, phone number, blood group etc.) but I managed it in the end.
For what it is, it should be free. I only bought it because I'm a fan of Novation stuff (I have a Nova gathering dust, amongst other things). There are plenty other ways, free, and 64bit, of emulating a TB-303... Afterall, it was an extremely basic analogue synth in its day. My main use of my TB-303 was to use it to drive my Juno and SH-101, back in the days of CV (control voltage).