I'll just emphasize what has been said before.
1. Automakers quantities allow customization at no cost or even cost savings. In working on my vehicles I have found many screws/bolts that I have never encountered in non-automotive applications. Some of the customizations ease assembly, and don't really have any challenging performance requirements. Unfortunately head bolts and suspension bolts are likely to fall in the special performance class. Another reason automakers may go to custom bolts is to reduce legal liability for off the shelf parts.
2. In the US the automaker is legally required to make replacement parts available for ten years after date of manufacture. If your car is less than ten years old, or if the identical design element was used on a vehicle in the last ten years the part will be available from the dealer. It is worth checking multiple dealers as I have encountered price differences over 300% on the same part from different dealers in the same city. The law doesn't set prices and different dealers have different theories on keeping customers happy and coming back to buy new vehicles.
3. Auto parts shops (NAPA, O'Reilly, Autozone, etc.) can usually access these parts, and larger stores often have shelf stock on commonly needed parts. This is great if you are fixing a Ford pickup or other popular model, less helpful if your car is much older or less popular. There is a great variation of the density of these shops, and I have noticed an inverse relationship to area income. Wealthier folk in general pay other people to fix their cars. If you are getting engineer or technician pay you may have to go far from home to find a good shop.
4. Online parts stores (Rock Auto, A-1 Auto Parts and others usually are far cheaper than the local stores, even after paying shipping charges and have far larger stocks and selections. Navigating to your part is sometimes tricky if you don't know the exact nomenclature, but your suspension bolts shouldn't provide too much trouble.
5. As a last resort, you or a local machine shop can manufacture bolts to your specifications. I have done this on several occasions, sometimes starting from a similar, but not quite right bolt. This requires you to fully understand the needs of the application and/or a willingness to enforce limitations on the vehicle utilization. Not cheap, but much less than the cost of a new car.