Mcmaster

Metric round tubes
10, 15, 20mm http://www.mcmaster.com/#standard-stainless-steel-hollow-tubing/=qe21r4

Inch - mm conversion

 * Small-Diameter Round Tubes—Unpolished (Cold Rolled)
 * 1inch = 25.4mm
 * OD            --- -   ID
 * 1/8"   3.18 mm 	0.0690"	±0.005"	8457K51
 * 3/16"  4.8 mm   	0.1315"	±0.005"	8457K52
 * 1/4"   6.4mm    0.1940"	±0.005"	8457K53
 * 5/16"	 8mm  0.2565"	±0.005"	8457K54
 * 3/8"	 9.5mm        0.3190"	±0.005"	8457K55
 * 7/16"	 11 mm        0.3815"	±0.005"	8457K56
 * 1/2"	 13 mm        0.4440"	±0.005"	8457K57
 * 9/16"	 14.3mm       0.5065"	±0.005"	8457K58
 * 5/8"	 16mm         0.5690"	±0.005"	8457K59
 * 3/4"	 19mm         0.6940"	±0.005"	8457K5

sa suppliers
http://www.stewartsandlloyds.co.za/PriceLists/tabid/103/Default.aspx

http://www.macsteel.co.za/products/carbon-steel-tube-pipe