What about this one

http://www.arceurotrade.co.uk/Catalo...-Collet-Chucks

Part way down the page, ER 16 on a 20mm parallel shank.

ER16's are good to 10mm
I suspect you have found out but they close a full mm so they will hold any size.

6 to 7mm will hold 6mm and 1/4", 3 to 3.5 will hold 3mm or 1/8" and 9 to 10 will hold 3/8" and 10mm so you only need about 4 collets to hold eith standard sized cutters.