そんで、いたの？ (= それで、いたの？) means "And, was (it) there?" "And, did you see/find (it) there?" そんで is a colloquial, collapsed way of pronouncing それで. The subject for the verb いた(居た) is left out.
いたじょいたじょ (= いたぞいたぞ) literally means "(It) was there, (it) was there." This is how 山田くん in anime ちびまる子ちゃん usually talks (e.g. 「～だじょー。」「～したじょー。」 ). I think young children tend to pronounce the sentence-final particle ～ぞ as ～じょ, though it could be a regional dialect.
"(I'm surprised that) You managed to go to the mountains in such a cold day. And, was it there? / And, did you find it there?"
"Ahaha, yes, it was there! / yes, I found it there!"
By the way, the よく means "manage to (do something difficult)", not "often." Eg:
What a surprise you managed to go.
How lucky you got back safe and sound!