Danielle, 

à or de can follow ‘ être adjective ‘. 

The preposition used after the adjective in personal ‘ être adjective ‘ expressions depends on the adjective, not on the verb or noun following the adjective.  

In impersonal ‘ être adjective ‘ expressions, the subject ( ce or il ) being ‘ real ‘ or a ‘ dummy ‘ subject is the determiner of the preposition used.

For instance ‘ Prêt à manger ‘ are practically everywhere in France ! 

See link below to Laura Lawless site for discussion and lists : 

 https://www.lawlessfrench.com/grammar/adjectives-with-prepositions/

 https://www.lawlessfrench.com/grammar/using-impersonal-expressions/

 https://www.pretamanger.fr/fr-FR/shop-finder/l/paris