Yes you can get it from mcmaster. in fact thats where I got it mostly because I had other things to get from there as well. although you can also try speedy metals based in Wisconsin. I use them frequently when I order material. most of the work I do is for a pharmaceutical customer and requires 304 or 316 stainless and the quantity is 99% of the time less than 5 pcs. so I rarely buy a length of anything. speedy is great on pricing for saw cut pieces. although they do have an "online" store front you can buy through. the pricing will always be significantly cheaper with them if you call or email the order in.

as for the polish on the valve faces. I have a bench grinder with a 3M deburring wheel on one end and a buffing wheel on the other. I lightly deburred the valves edges from parting them off and then ran it across the buffing wheel after I shut off the power to slow the wheel down. the deburr wheel greatly reduces the amount of hand deburring on pretty much anything. cannot live without it.