You currntly can’t search pokemon by level in pokemon go, but !
here are some tip that help you can use in pokemon go search by level :
type candyxl in the search box to see pokemons that are above level 40 !
make a level 50 tag. tag there all the pokemons you leveled up to 50.
then search this tag when you want to use level 50.
and then sort by cp and you can see the highest pokemon level of each pokemon.
if you see xl candies in the power up button that pokemon is level 40.
and you can use apps that will show you the exact level of the pokemon.
based on cp and the little circle on the arch above the pokemon.
you can use poke genie or calcy iv (those are best 2 apps to see level in them)
or just search “pokemon go level” inside your apps store.