Our cot mattress was from IKEA and it is really nice! It was the 60x120 one not sure if they do the same in your size
I have had all my mattresses made bespoke for the next to me, cribs, play pen etc as I have never been super happy with the quality of what comes with it or they are so expensive. I have used this website several times, they are great: https://babymattressesonline.co.uk
We got ours from mamas and papas and have been really happy with it
I bought ours from Amazon. I think it was mother nurture brand. Have tried the pocket spring and the premium, bub sleeps well on both x
Mamas and papas mattress for us, just check the size as cot and cotbed mattress sizes are different. I think I sized it up and it would fit an ikea cot but we got a great deal so stayed m&p
I’d check on Amazon, they have some at reasonable prices:)