Small boilies in winter?

This past weekend I had 4 specimen rigs in the water. 3 of them baited up with double 15mm boilies. The 4th one had double tigernuts. I have not gotten one bite, yet the fish were very active on my single papgooi rod I had out.

That made me wonder if the fish don't perhaps prefer smaller baits when the water is getting colder?
Where can I buy 10mm and 12mm boilies of various flavors, or even plain in order to flavor them myself?

That's the difference between specimen and papgooi I suppose - tempo vs size. But yes it could be that the fish in that particular dam do feed on smaller baits when it get's colder - their metabolism slows down and they may just want to pick on smaller offerings here and there. You can also just cut your boilies up a bit to try get them smaller - shave it off a bit, or if they're hard enough then even cut them in half. The notion that they have to be round is rubbish. Squares, discs, barbell, cylinder shapes all work.