Dr. Leiman writes on the mouse that is 'half-mouse-half-earth" that, (p.452):
"... it comes as no surprise that the rabbis discussed the status of a creature they had never seen, and one that modern scholarship would label as imaginary. The greatest scientists and historians of their day took its existence for granted. If so, its halakhic status needed to be discussed and clarified."
In the same articles Dr. Leiman quotes Sherira Gaon, Maimonides, Abraham Maimonides, and R. Samson Raphael Hirsch, all of whom admit that, to some degree, the Sages worked with the science and mathematics of their times, and that things they state in those fields should be considered in that vein.
Mermaid legends in the Near East began as far back as 1000 b.c.e. The Sages heard of such a creature, and felt that they needed to discuss it during this veterinary gloss in Bechorot.
So to answer the question, just because the Sages discuss a creature doesn't mean it exists, and can be found. It is possible that mermaids don't exist, and can't be found anywhere, and that the Sages were working with bad intel.