I got these for my first from Amazon, worked great and will be using them again this time round. https://amzn.eu/d/0brMs3ba
We’ve got the Tutti Bambi ones and just double checked the dimensions of the mattress etc Tutti Bambini Bedside Crib Fitted... https://www.amazon.co.uk/dp/B0BS3Z8MCQ?ref=ppx_pop_mob_ap_share
The crib sheets from mothercare in boots are a perfect fit, they’re £8 for 2, doesn’t say maxi cosi iora air but the mattress dimensions were the same and when they arrived they fitted perfectly. https://www.boots.com/mothercare-grey-bedside-crib-fitted-sheets---2-pack-10294472
We got the ones from maxi cosi but I would not say that they were reasonably priced! You could try a slightly bigger sheet and try to tuck underneath?