Sells or sold?

The core of the sentence is "I find a store." The rest is a relative clause that says something about the store. The two can have independent tenses.

"I found a pet store that sells ferrets" implies that the store sells ferrets in the present.

"I found a pet store that sold ferrets" implies that the store has sold ferrets in the past but does not necessarily sell them now.

Since you want to "change the sentence to Simple Past Tense," I would put all of the verbs that are currently in the present tense into the past tense: "I found a pet store that sold ferrets."

I disagree with Rik853's comment that "I find a pet store that sells ferrets" is wrong. If the writer is speaking in the present tense, the phrase is grammatically correct. It is no different to saying "I find a stone that has holes", where "sells" and "has" are the present tense of the verbs, "find" is a perfectly usable verb.

"Sold only if they used to sell them but they do not sell them anymore." Well, if you were telling about something that happened today, that's true.

But if you were telling a story from the past, you'd say, "I found a store that sold ferrets." And that doesn't mean that they no longer do so.

Yeah, i agree with Rik853,
"Sold only if they used to sell them but they do not sell them anymore."

I find a pet store that sells ferrets is wrong.
I found(past tense) a pet store that sells ferrets is right.
Sold only if they used to sell them but they do not sell them anymore.