Carpet can be ordered in any length that you require, it is the width that only comes as standard.
It is a pity that carpet is not available in the dimensions of your room as obviously, having only standard widths leads to joins and wastage.
It is always best to get an experienced carpet fitter to lay a new carpet for you, as they will be able to work out where the seam should go for the least chance of visibility which is also helped by laying the carpet all in the same direction.
If you have a wide room, it is always worth asking if the carpet comes in a wider roll. Some shops only use the smaller width rolls but are able to order the larger ones and this will potentially save you money as there will be less wastage.
Ideally, if you are going to use the services of a carpet fitter, many will measure your room and order carpet for you as they will know how it can be cut and fitted to get minimal waste with minimal cost. Just make sure the fitter is trustworthy if you choose this method.