Measure the amount of space you have to work with ahead of time. Then, you will know the options that are at your disposal when you are out shopping.
Visit more than one mattress store in your area. While price shouldn't be the most important factor when buying a mattress for a child, you still want to be able to get the best deal possible.
Look for a firm mattress. Remember that your child's spine is still in development, so the amount of cushion an adult can handle might not be safe for a child. However, the mattress shouldn't be so rock-hard that the springs can't shape to their body.
Find a mattress that your child can grow into. While you don't want to move a child out of a crib before they're ready, moving a child straight from a crib to a twin bed may be an option if you take the proper safety precautions.
Consider all safety measures with the new mattress. If you are moving your child into a twin bed at the toddler stage, you may want to consider buying rails for the side of the bed. If you are buying a mattress to use in a bunk bed, make sure it's lower than the bunk-bed railing.
Bring your child with you to test out the mattress. While they may not know exactly what to look for in a mattress, they'll probably let you know if anything stands out. And test it out yourself while you're at it.