I can't help you with specific carpets but I am sure you are aware that tent carpets are made to fit a specific tent. However, if you measure the floor space you wish to cover then check out the floor space of other tents you should be able to find something that fits.
Much easier of course if it is a rectangular or square area rather than shaped but bear in mind that you don't need to carpet right up to the walls of a tent... firstly you don't walk right up to the walls so why cover it and secondly you probably have gear alongside the walls which doesn't need a carpet underneath.
So, you should be able to find something or have you thought about waterproof picnic rugs which are very similar to tent carpets?
The vango inca 600 carpet is 420 x 310, which I think is 20cm and 10cm bigger than you need on the dimensions. You could alter it, but seems a lot at £58 if not a perfect fit...also not sure of quality of these, and it's grey! But gives you an idea. As your living area seems to be a simple rectangle it might be cheaper to stitch some large picnic rugs together.