Cracked alloy on 2000 Mk 2.5

[Aeroscreens]

Anyone experienced like this before? 2 weeks ago there was defo no sign of damage and now this crack which sadly can also been seen clearly from the rear of the wheel. Mrs A doesn't recall hitting a pothole and there's no other obvious signs of damage. Guessing an age related problem? I assume that the wheel is toast and will probably look to buy a 2nd hand set of alloys for the Mrs car.

Wheel is marked as a 15x6JJ and stamped F6. Recommendations please for a suitable set of cheap alloys to replace existing set...