You don't say how old you are, but for under-16s there's an interest-free loan scheme which bassdirect take part in: https://takeitaway.org.uk/
Otherwise, you're down to one of the following:
* Bank loan / other credit
* Sell a lot of / some expensive stuff
* Upgrade your current bass, one piece at a time
* Accept that it's a long-term (10+ years) goal and start saving.
* Train in financial services, law or data analysis, then get a high-paid job and go buy the bass for cash. In your Ferrari.
In all seriousness, it's way cooler to see someone who can really play on a ghetto instrument than it is to watch someone get by on an expensive one. Some of my favourite musicians make a point of spending as little as possible on their instruments.