If you have a toddler bed, a smaller bed made specifically for the toddler stage, you can continue using your crib mattress. However, crib mattresses are quite firm, as this is a requirement for infants, so you may wish to consider a mattress more targeted to toddlers.
Some people choose to skip the toddler bed and move straight to a twin bed. Any twin bed mattress will work for a toddler provided it’s firm enough, but you should look for one that’s better suited to children than adults.