"There are" is grammatically correct. The verb and the object should agree in number (both are plural).
"There's" can be a contraction of either "There is" or "There has", neither of which is grammatical in this sentence.
IMO the most likely reason for "There's over 100..." was just a mistake in spoken English. The speaker said "There's" before realizing that the object of the verb was plural.
FWIW There are some rarely used contractions like "when're", "which're", "what're" for "when are, "which are", "what are", etc. I suppose "there're" would fit that pattern, though it is hard to pronounce and doesn't save any space in written English.