I don't believe that individual token markers are tagged for searching, only complete sets. There are a number of sets that do contain concentration markers. The easiest thing to do is to filter the list by Token Markers, and then look for the term "Concentr"on the page of any that you like the look of. Here are a couple:
Easy to Read Token Markers - My set
Token Markers: Pack 1 - By Dawn Caroll
You can also just look over token marker packs and choose any marker you like that looks like Concentration to you. You don't have to use them as labeled.
Hopefully, some of the other pack makers will chime in. Those are the only two I could find with the quick search method I used.