www.ribbonmoon.co.uk has some beautiful buttons, lots to search through though. One of the shops on Sharrowvale road (the front room?) sometimes has nice buttons or you might find something in a charity or 'vintage' shop that you can extract the buttons from?
I have a strange obsession with buttons Places to get them from
The stall downstairs castle market have quite a big range of buttons very friendly so if you need any ordering im sure they would help.
Fine fabrics hillsborough have a decent amount of button not sure if they have the ones you require as i havent looked recently. They might be able to get you some if not as they have another shop too
Also try ebay they are loads of ebay shops selling pearl type buttons search pearl buttons under crafts some lovely ones come up and reasonable prices!