The stock allens work so HD will have some. If you want them cheaper and don't feel confident to ID them yourself, take one to a Hardware store (NOT Home Depot) and have them see. They should have a bolt guide there to do it your self too.
They are allen bolts, but don't get the usual "black oxide" as they will rust immediately outside. Get zinc plated or stainless and use a tiny bit of anti-seize. The load on them is negligible.
Do NOT over-torque them! It's less than you think. Spec in the manual.