Where do I get extra parts as in bolts nuts etc.?

After putting most of the mechanicals on the x carve together, we are missing some 5mm bolts. Can I just get these at the local big box?

Yes you can. Customer support will send them as well

Big box stores will have some basic M5 fasteners. Some cap screws, stove bolts, maybe a countersunk, in a few different lengths.

Find and get yourself acquainted with the local industrial fastener place in town, if you have one. They will have button heads, socket heads, countersunks, low profile, whatever you want in 2mm length increments. And often they will have screws and bolts black oxide And at quarter the price of big box.