An important factor to consider when shopping for crib mattress is the firmness. You need a firm mattress, soft ones are a safety hazard for the little ones.
You will also have a choice between foam and coil mattresses. Coil mattresses are usually more expensive and more durable. Usually the higher the number of coils the more durable the mattress is. You can buy coil mattress anywhere from $ 35 US and up.
If you choose foam, make sure it is not soft. The more layer on the mattress the better.
Crib Fitted sheets should be 100% cotton for allergy reasons and fit very tight.
A word about crib bumpers. They are very cute. I spent a lot of money on them with my first child. However, if you need to save money, then it is not a necessity (assuming your are using cribs that meet today’s safety standards). In Canada, the health organization advises against using them because of the chance of suffocation. However, you can get crib skirts separately.