For those that might need a round ball mould I wanted to give a recommend to Jeff Tanner in the UK.
http://www.jt-bullet-moulds.co.uk

I ordered a .750 ball mould for my 12bore J. Blissett & Son full rifle, the mould was at my door in Colorado, USA within 10 days!
Easy casting mould and makes very nice consistent balls.

I recommend his moulds and service.
- Mike

EDIT: similarly to Daryl, I use NEI Mould Prep (colloidal graphite solution) on my moulds.