Can you say A found... that house was a found... it was a real bargin. Or is only A find correct here? Is there any instance that you could say "A found"
The word is "find" which, as a noun, refers to something (usually good, exciting, valuable etc.) that has been found. Archaeologists keep lists of their finds.
I don't think so. Merriam-Webster lists only one definition of "found" as a noun: "free food and lodging in addition to wages" and it's not related to what you mentioned (i.e., being "a real bargain"). If I understood their definition of "found" as a noun correctly, it could be used in a sentence such as:
You'll get 100 bucks and a found for the job.
Perhaps saying "That house was a marvelous/amazing/incredible finding!" would do, but this doesn't sound very natural to me. Maybe someone who knows better can chime in with a good expression. In any case, I strongly believe that "a found" would be incorrect in the phrase you stated.