Wolfgang in Vonitsa is very good but expensive when we last checked. There is also Phil in Nidri. I no longer have their numbers but if you ask the same question on the ybw forum then someone will come up with contact details for them.
Geordie Jan the welding man here, I am currently building my own "stern management system" from 1 1/2 inch 316L tube and would be happy to have a look and advise on your framework. I carry 3/4" - 1" - 1"1/4 and 1"1/2 316 on my mobile workshop along with tube bending rolls and tube bending dies.
UK mob tel 0044 (0) 7949 267787
Greek mob 0030 698 7151949 email@example.com