Correct spelling for the English word "thems" is [ðˈɛmz], [ðˈɛmz], [ð_ˈɛ_m_z] (IPA phonetic alphabet).
The word "thems" is actually a contraction of the pronoun "themselves". The etymology of "themselves" can be traced back to Old English, where the pronoun was composed of the word "them" (a dative/accusative plural form of "they") and the Old English reflexive pronoun "self". Over time, "themself" evolved into "themselves" and eventually contracted to "thems" in some dialects.