I'm trying to catch entei and raikou but they keep fleeing
No, there are no items that will do that. However, there are moves and Abilitys that can make them unable to flee, like Mean Look and Suction cups.
pokeslash is half right mean look works but ones the user is koed it is no longer useful. Suction cups only prevents the Pokemon with the abilty from being switched out by moves {whirlwind} . but arena trap ability will prevent escape I think only trapinch has this ability