General substitutes:
* Artificial
* Fake
* Imitation
* Synthetic
* Substitute
* Counterfeit
* Mock
* Spurious
For specific types of substitutes:
* Substitute (for food or other products)
* Knock-off (for imitations of brand-name goods)
* Copy (for a replica of something)
* Simulant (for a substance used to mimic the properties of another substance)
More informal synonyms:
* Phony
* Bogus
* Cheap
* Second-rate
* Inferior
The best synonym will depend on the specific context in which you're using the word "ersatz." Consider the nuances of the situation and choose the word that best reflects the intended meaning.