heres the one i pointed out to him
http://cgi.ebay.co.uk/Thorite-Diamon...item518d2661a6
as i said, not actually personally tried it so can't comment on its quality etc. but for starting out or even just to add another workhorse to the arsenal i'm sure you can't go wrong.
.5 gravity brushes seem quite rare. everyone is obsessed with 'detail' and having a smaller needle/nozzle setup.