I am not sure if the 'あ' being used instead of 'は' or there is a meaning in the word 'そいつあ'.
In addition, is it possible that the 'あ' is a kind of particle or suffix?
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「そいつあ」 is a colloquial pronunciation of 「そいつは」. This is most common among male speakers around Tokyo in their informal speech. It is not something they would use in school or business.
Particle 「は」 is often pronounced like 「あ」 in other areas as well when combined with certain words in informal situations. For example:
「それは」("That is ~~.") ⇒ 「そりゃあ」 or 「そりゃ」
「たまには」("Once in a while") ⇒ 「たまにゃあ」 or 「たまにゃ」
" is it possible that the 'あ' is a kind of particle or suffix?"
This 「あ」 is not a suffix. It is, in my own words, a cross between the lazy, colloquial pronunciation of 「は = wa」 and a "filler syllable" when one wants to omit a 「は」. This may be a terrible way to describe it but that is what I honestly feel when I use this 「あ」 myself as a Japanese-speaker. I feel I am half-using the particle and half-not-using it at the same time.
Point is everyone will understand what I am saying, which is the only thing that counts in colloquial speech.