We had a mattress made to fit our travel cot from the local market, as our LO seem to struggle to sleep well on the hard board wraparound mattress, Since putting the mattress in which was made to measure to fit cot, LO has slept better in cot when we use it. Think we paid about £15 for it.
Alternatively I believe you can get mattresses for travel cots from places like Mothercare.
Supplementary mattress for travel cots available in any size.
Hypo-allergenic foam mattress 7 cm. depth.
Made from firm CMHR25 density foam.
Not all folding mattresses follow traditional . Some folding mattresses are intended for children, and may be smaller than a twin. Others simply come in irregular sizes. Check the measurements before purchasing a mattress if you’re not sure.