Yes cookie cutters are easy to buy from flooring wholesalers. I have two an Orcon and a Roberts other makes are Crain,Wolf, and Gundlach
Bear in mind that are deseigned to be used from the back of the carpet and not through the face fibre as is generally practised.
As Derek has said either retuft if a small area or cut and plug (bonded insert) Carefully row cutting both the field and the plug will be more likely to produce a good match than using a cookie cutter from the top.
Michel