Yeah, you'll be hard-pressed to find high strength flange-head bolts at Lowe's/HD. Ebay, Bolt Depot, BelMetric, or Amazon are your friends
.
You will need the longer M12 bolts if you want to retain the tie-down/tow hook brackets. As for the other (SAE) bolts, AOR shipped the bumper attached to the cross-member with these bolts, but they didn't line them up properly and thus cross-threaded the bolts. Ugh. I did have to run a tap through the female threads, and get the new bolts. Hopefully your bumper will be better assembled than mine was.