In the United States, at least: There are retail stores - brick and mortar, I know! - where various “beauty supplies” are sold. Find one where they sell hair bleach etc. They will also sell hair shears.

I got the second-cheapest set they sold, for around $25. For our amateur purposes, these student-hairdresser-grade shears are more than adequate.

I cut my own hair probably 6-9 times a year, and those shears are still the sharpest things I own, fifteen years later.

(Shears are actually really specific cutting tools. Mine have only ever cut hair, which I credit for their long life)