Replacing a corroded bolt

I've been having issue with my 2012 Prius, the whole engine shutter thing. So I did all the recommended stuff, clean EGR valve/pipe/cooler, replace PCV valve, replace spark plugs, and clean intake manifold. I forget the name of the part, but I believe it's the air intake system, right before I remove the intake manifold. Anyway, two of the four bolts are corroded. One actually broke. They are M6-1.0 x 50mm flanged hex bolts and half threaded. Oddly, I'm having a hard time finding them locally. I have only found some fully threaded ones at Home Depot. I haven't asked the dealer but now I'm wondering if I should replace them with something sturdier. There was a lot of oil in there that I had to clean out and I suspect that caused much of the corrosion. I guess it was Zinc plated because it was white. Any suggestions? Should I see what the dealer has? Find something more corrosion resistant?

 

Yes, I believe that's it. I did notice the difference as the NutzAndBoltz video had the two and tow while mine had four. Probably a good idea. This things had been taken apart for far too long as it is. Thanks.