Tim in Belgium

I need a matress for my rental place, super king size, nothing expensive as my time here may be limited. So where can I get a good value/comfy one from that doesn't cost through the roof? Online or real shop/chain.

- And there is no Ikea near me (I'm in Reading at the moment if that helps).

I've no idea having not had to purchase any furniture ever in the UK.

Cheers,

Tim