In all honesty, if I go to a garage I'd rather use the parts that they provide to prevent exactly this from happening. Getting into fights is more stress and time than the money you save however, if I ask to use original parts and the garage says that he can`t get it I will usually ask the garage if it's ok for me to bring him ORIGINAL parts, in which many cases they don`t have issues with. If I buy my own jobber parts it's because I expect to do the job myself. The fighting and aggravation is not worth the savings. Now in terms of what happened to the OP, I laugh at the response. Bolts don't just loose by themselves especially after only 600kms. If they didn't fit properly the garage should have said these bolts are garbage, I can order you better ones. He didn`t. The only reason the bolts would get loose is if he didn't tighten them properly. And for those who say they've never seen their mechanic use a torque wrench, well it`s because either they didnt have to on your car yet or maybe your mechanic is not working properly.
When you install a exhaust manifold or headers or anything near the engine you need to use a torque wrench for 2 reasons, 1) so that you don't overtighten the bolts and either snap the bolts or crack the manifold 2) it being so close to the engine the vibration can loosen the bolts so if they're not tightened to spec you wll get exhaust leaks and risk breaking something from the vibration when it comes apart like it happened to the OP.
For all of you arguing about bringing your own parts, not to bring your own parts, it is pointless. The mechanic accepted to do the job, didn't do the job properly, didn't warn the OP about the risk of the bolts loosening up, Either he fixes the issue or he doesnt. If he chooses not to hopefully the OP has a bill (didnt pay cash) he can go to small claims since the labor is guaranteed regardless of the provenance of the parts. The mechanic accepted to do the job he assumes the risk.