As butchers, throughout the Kingdom, usually had sawdust on their floors it presumably came from sawmills or factories that were in the woodworking trade. It was bagged, when created on a large scale, from the cyclones that extracted it from workshops. I suspect it would be of varying grades and maybe dependant of source of supply.
The Co-Operative movement, in those days, for instance, made furniture for sale in their larger stores. Small companies and one-man bands, I imagine, bought it from wherever it was available and at reasonable cost.
There are old style butchers usually in small towns, who use traditional methods of display, in the manner familiar to older people. I have not seen any with sawdust on the floor but I believe there are still some in the 21st. century.
It was not simply to absorb spillage from carcasses etc. but the sawdust also acted as a de-humidifier.