Mike, do you mean that Sheffield has 416 plate? Not that I need any, but Rashid might.
Rashid, I can't really give you a good answer to your question about 303 stainless.
Yes, 303 solders just fine. One thing is that most makers want their pins to be hidden, and matching 416 pins to 416 guards or bolsters makes hiding the pins very easy.
I've used both 416 and 303 and had no problem other than hiding pins in 303. I didn't really know what alloy the pins were made of.
Industrial metal suppliers to manufacturing facilities will probably carry more 303 than 416.
I have two local steel suppliers here in town. I suggest you check for similar suppliers in your area. Mine would be concerned about quantities and shipping.