Ive been purchasing on line from
cpc.farnell.com/c/electronic-electrical-components
for several years especially since the demise of Maplin. To be honest, I think this heat shrink should be priced per piece not per pack but if you need these sizes.
The downside is you need to spend approx £15 exc vat to get fee postage but they do sell a vast array of tools electronics, banana plugs etc.