I see this question posted over and over and over again in rat groups. People get so worried that their rat's teeth will overgrow if they do not have something to chew on and wear them down.
The truth is that rats DO NOT need anything to chew on. They very naturally grind their own teeth together to wear them down. This is called bruxing.
Over grown teeth in rats is called malocclusion and is either caused by injury or a genetic issue where their teeth do not align properly. Without being properly aligned they can't grind them down.
When a malocclusion occurs they have to have their teeth trimmed for the rest of their life.
Read more about malocclusion and find a guide on how to trim a rats teeth on the rat guide website.
Sadly there are some lines of rats that are very prone to this, such as some marble lines.
All that being said, it doesn't mean things to chew on are worthless. Things can be great fun mental stimulation.
Stay away from stuff like raw hides, which are not even safe for dogs! Most wood block marketed for small pets are fine. Chicken bones are safe for rats even cooked. While they are very deadly for dogs/cats because they can splinter and/or be choked on, but rats gnaw things down making them just fine. Cardboard is also usually a fun thing for them to chew on.
I usually toss in cardboard rolls & boxes for them to shred lol
One of my rats favorite things to chew is called a Knot Nibbler. It is like alittle stick puzzle they can pull apart.