Hi Matt,
Can't help with material supply but this may help you source something suitable if you go door knocking. Approaching a stockist for a bar end or off-cut may work too.

Stainless at this size is usually bar - I've not seen it hollow. And usually it is still Inch sizes 2" - 2 1/2" - 3".

Unless you're submerging the item in sea water then '303' will be good enough - 303S31 to be exact. It's the common free machining stainless steel. It should be austenitic which means it doesn't harden by heat treatment, and is usually non-magnetic. If you knock on a tool room door then take a magnet with you. Slight magnetism can occur from the inclusion of cobalt in some manufacturers supply. Use a piece of carbon steel as a reference. Off-cuts usually are missing identification paint or codes.

Possible sources.
http://www.glrkennions.co.uk/brass-b...StainlessRound
http://www.steelexpress.co.uk/non-fe...ess-steel.html
http://www.mkmetals.co.uk/catalogue/128/1


BTW Sulphur makes it free machining, but sulphur inclusions or sulphur strings can make it crack - if it does there's no way back. I've seen that half a dozen times in fifteen years but then we machined several hundred tonnes per year.

Good luck
D