This information provided below is on the assumption that you are not going to put this inside a person.
I am assuming it HAS to be a glass encapsulated chip
Does it have to be Magic S50 like in the xM1?
You also mentioned NExT and xSIID, so would a xNT equivalent be suitable ( NTAG 216 ) , it is still ISO14443a.
You could look at something like a cheap pet chip from aliexpress
Normally, they are the FDX-B 134 kHZ chips, but they also do a NTAG216 as mentioned, here’s a link
The biggest difference between the xM1 and the xNT
xM1 - changeable NUID
xNT - NDEF data storage and transfer
I’ll have a look and see if I can find any xM1 equivalent, I have never seen one before, but that doesn’t mean they don’t exist, also I have never looked for them before.
If you get some, do not put these inside a person.
It is a typical Aliexpress listing, so even though it implies you will be injecting your pet seagulls and pigeons with 134.2kHZ chips
It looks more likely, it will actually be the NTAG216 as mentioned
Protocol ISO14443A, NFC Type 2
Chip type Ntag216
Material Polypropylene, Biocompatible Glass Without Parylene C coating (NFC microchip)
Sterilization EO gas, 5 years’ guarantee
Bar code Contains 8pcs adhesive-sticker