went to real and charity shops requesting bags of craft buttons - only had one success (British Heart Foundation), but a second (Salvation Army) said 'not at the moment'. So they do, but others are also on the lookout! Oxfam seem to card theirs up - and Age UK South Lakes had sets too, but for something taking up to/at least 55 buttons* (and three beads) (and a little red heart I didn't even know I had!), I need cheap and plenty!
I know what you mean about needing lots of buttons when you have a purpose - I had so many buttons I gave a huge box away. Now I am regretting it because I am searching for buttons suitable to put on the dolly clothes I am making and will have to go out to buy (little fingers need biggish buttons at this stage so old shirt buttons won't do)