Replacement Sonalerts (or equivalents) may be found on old PC motherboards. Since DSP RDs are at TTL levels, the 5/6 V units on motherboards are nearly perfect. If in doubt, check the DCR on the old one and the same on the candidate. ~20% variation should be fine.
Also, the speakers from DELL latitude laptops may be good for non Sonalert designs. As ususal YMMV