I was thinking more of ER20 just for pen turning similar to those sold by Axi.
http://www.ebay.co.uk/itm/ER20-MT2-M10-CNC-Milling-Collet-Chuck-Holder-Lock-Nut-Outer-Diameter-28mm-/301728498683?hash=item46406b83fb:g:9OQAAOSwDNdV5R1K
If you want something bigger then there are ER32 MT3 for about a tenner on there.
If you go up to ER32 or bigger though the cost of the collets triple.
I've already got a set of ER32 collets so I'll probably try and get a MT2 ER32 but for some reason this combination is more expensive at about £18?!