I'm very sorry for your budgie's passing...
Rest in peace, Candy.
It really wasn't a good idea to take your budgie to the pet store, because while in there she may have caught an illness from the other birds, she could also already have been ill since her previous mate had recently passed away.
In case you don't know, when getting a new budgie, quarantine measures should be implemented in order to avoid the potential spread of illness the new bird could be carrying.
For detailed information, check this link: https://talkbudgies.com/new-budgie-ar...necessary.html
Whether your budgie got sick from a cold or something else, when a pet shows signs of being ill, the best course of action is to seek medical care from a vet, in this case, an avian vet professional for an accurate diagnosis and treatment plan.
Given the circumstances, it would be best to have your remaining budgie, Fairy examined by an avian vet. The cage and all toys/accessories should also be thoroughly washed and disinfected.
Despite the very sad circumstances, welcome to the forums.