Hi Hogster,
I've been away overseas for a bit so haven't had much time to get on the forum lately.
I've bought a fair bit of stainless tube from Tubesales down here in Melbourne. I've bought 4" from them in the past so there's a good chance they have 4.5"
Here's their web page:
http://www.tubesales.com.au/index.htm
3mm wall is way overkill. Why so thick? They are going to weigh a ton.
Cheers