If you can find a yard that has old Buicks from the early to late '60s, you can find a buzzer. A lot of the old Buicks had speed warning speedos. Also, unless you are going for a 100% correct '69, any buzzer from a GM car can be used. The buzzers were used for seat belt, head light, door ajar warnings. The speedo needle contacts a wire on the warning needle. This contact completes the ground circuit to the buzzer. It's a very simple circuit and a harness is easy to make. Unless you are trying to be 100% correct you can get your speedwarning up and going for very little expense.
There is a yard near me that has many old Buicks with the speedwarning speedos.