If you still have the old carpets take them to a carpet suppliers who could use them as a template to cut and edge some offcut or end of roll which will work out cheaper than buying from a caravan dealer.
If you don't have the old carpet make some templates from cardboard boxes opened out and gaffa taped together.
We got our replacement carpets from a company called Kustom Sport (http://www.kustomsport.co.uk/) who have templates for many caravan models. Not cheap, however as ours cost £200 but there was a huge range of carpets to choose from and the quality was far better than the originals.