The Google Ngram Viewer shows at first glance is much more frequently used than at a first glance.
Furthermore, as in this link(OALD), at first glance is entered as a stand-alone idiom. So I can conclude at first glance is the common one.
However, in terms of grammar, I don't see any problems with at a first glance and, rather this feels more correct.
Can I use at a first glance instead, and would it be accepted as natural and correct in native-English-speaking societies?