You don’t need a granite stone, we’ll at least you don’t need to invest in a super flat one.
Get large format glass or porcelain tile from a big box, or find a company that installs counter tops and does the cutting for stone counters. Ask if you can have an offcut piece of quartz counter top.
Having a cheap, flat surface is great for lots of tool sharpening, and it definitely does not need to be expensive or an investment.
If you don’t have adhesive backed sand paper, you can also get a can of spray adhesive. Doesn’t need to be the super strong stuff.