Plurals of "infimum" and "supremum"

Solution 1:

For mathematicians, the plurals of infimum and supremum are infima and suprema, respectively. Google Ngrams shows that the incorrect plurals infimums and supremums are used roughly equally often (and much less often than the correct plurals), so I don't believe there is actually any asymmetry.

Here is the Google Ngram with the incorrect plurals:

They are used so infrequently that they barely show up on an Ngram if you try to compare the usage of the correct and incorrect plurals.

Solution 2:

According to this dictionary of math terms, the plural of infimum is infima. Similarly, the plural of supremum is suprema.

As this NGram shows, suprema and infima are also used more than the plurals formed with an s.

Solution 3:

The already given Ngrams are kind of biased since suprema and infima appear as plurals outside of mathematics. One can argue that this is the plural in general. But more than that, those words also appear in inner Latin, Italian or Spanish texts (ex. Corte Suprema).
I suggest Googling the books on the subject "mathematics" for the period 1950-2010.
The results show:

• infimums 457 results
• infima 5590 results
• supremums 708 results
• suprema 8940

In conclusion, I wouldn't say supremums, infimums are incorrect, but are less used. You can't go wrong with infima/suprema. Note that Wikipedia math articles, such as this also use infima/suprema.