What is the best product to get rid of old dog urine stains on carpet?

Natures Miracle available at pet stores

Vinegar

Try this:

http://www.instructables.com/id/Year-Old-Carpet-Stains-GONE/?amp_page=true

The best product may be one you make yourself out of items you have in your pantry. This site has 3 separate "recipes' to make to get rid of old dog urine stains! Good luck! http://everydayroots.com/pet-stain-and-odor-removal

vinegar and regular table salt. Pour white vinegar on the spot until it is saturated. Then pour the salt on top of the vinegar until you have a dry mound of salt. You will have to leave the mound there for a couple of days to let the salt absorb the vinegar and hopefully the remainder of the urine. Then vacuum up the salt. This worked for me on white carpet. Good luck.