Discontinued hardware

What do i do when the bolt i need for a car is nowhere to be found online or nearby? How can I find an alternative that won't risk breaking in a critical part like control arms or subframe mounts?

Have someone with a lathe make you a bolt (perhaps by modifying an available bolt). Or redesign the joint to use available fasteners.

Other than a whitworth thread fastener, I'd be surprised if no-one can supply what you need.

I assume you've already tried your local hardware store and, if applicable, fastener specialists.

Failing them, there are many specialist suppliers on-line.

What exactly are you trying to source - length, thread, tensile rating, head,etc and what is it fitting?

So far its the bolts for the rear control arms on my 2003 audi a4 that I'm looking for. Have an order placed but one of the main ones I need stated "Fits models from 2004" meaning its 04 and up so I'm gonna attempt it but expecting it not to fit. Really meant more of an in general what would be a decent process to follow to find specs if the bolt isnt labeled or is too corroded to read, or should i just get the strongest fitting bolt i can find and go off feel for torque to yield and hope? Might've been a stupid question this is my first car

There are standard torque specs for each size bolt, that you can easily find online. Have you determined the diameter, thread pitch and grip length required?

Here is a good description of how to do that:

https://www.essentracomponents.com/en-us/news/solutions/fastening-components/how-to-measure-bolt-size

Most industrial fastener suppliers have vast catalogues and if they can't direct irder usually have low volume custom high tensile bolt manufacturing capabilities.