Can I still make an indemnity claim if the seller's corporation has been dissolved since closing?

It becomes considerably harder, but not necessarily impossible. Once a corporation is dissolved, it generally ceases to exist as a legal person, which complicates bringing or continuing a claim directly against it, since there is technically no longer a party to sue in the ordinary sense. Ontario corporate law does allow a dissolved corporation to be revived in certain circumstances, which can restore its legal existence and potentially allow a claim to proceed against it as if dissolution had not interrupted things, but reviving a corporation is itself a formal process with its own requirements and is not guaranteed to be available or straightforward on every set of facts.

This scenario is exactly the kind of collectability risk that leads buyers to build in protections beyond just an indemnity clause against a corporate seller, such as personal guarantees from the individuals behind the corporation, or a holdback or escrow that already holds funds rather than depending on the seller entity surviving. A buyer facing this situation should get advice quickly on whether revival is realistically available and what other recourse might exist.