Credit cards, overdrafts, that kind of thing.
I bought my first car with the money I earnt working every hour possible during summer holidays between my 1st and 2nd year at uni (night shifts at a factory for 10 weeks, 6 days on, 3 days off, although sometimes worked through the 3 days off so worked 15 consecutive days).
Earnt a decent amount though. I also took out a 2nd student account so I could use the overdraft to pay for insurance, and somehow managed to afford petrol and running costs out of my student loan.
My 2nd car I was lucky in that my parents had come into some money and gave me a fair sum towards the purchase price, otherwise I'd still be on my first. They've filled up the tank 3 times for me in the last month, but on top of my own journeys I have done a lot of running around for them over the summer.
So, in short, I used a combination of all the available options - work, debt, and parents. I'm sure a job would probably work as well.
The down side is that I've just graduated and on top of my 12k or so student loan, have 3k on credit cards and am about £1500 overdrawn, but luckilly I've secured a decent paying job and the starting bonus will clear my credit card and overdraft debts.