I don't want to pay up for a DC-DC unit until I can see the rest of it working. So I've been using a PC 500w ATX power supply for the 12v that my BMS needs. It may be enough to run the motor controller too, with the back wheels jacked up so I can see them spin. After 2 years of dragging on I would like that.
Unfortunately I mislabelled the outputs so I've been struggling to get the BMS to run on 5v without knowing; the manufacturer politely told me that it's very hard for them to troubleshoot remotely so I ended up doing what I should have done first, checking everything with a multimeter.
I found the 5v problem soon enough, if I'd done that in the beginning I would have been two weeks ahead of where I am now. The reason it eluded me was that the main module and LCD display seem to run fine off 5v, despite being labelled 12v. Only the data collection modules really need 12v, and they were the only parts not working, so I didn't think to check.