We source our letter labels from Gresswell. They come in individual letter boxes of 250, and we understand that most schools do not wish to buy 23 (no need for Q, X or Z) boxes of labels. Plus, a full set currently costs around £160.
So, if you need a letter label top-up, we can send you one of our letter packs. These contain 600 letter labels (with more of the more frequently-needed letters) plus a box of 1000 label protectors. We charge £35 per pack, which includes staff time building the pack and postage.
If you'd rather buy direct from Gresswell,
search for 'individual alphabet labels' and 'demco label protectors' on their site.
(make sure you select white labels, and choose the smaller 35x25mm label protectors)