Ms James was found dead in woodland near her home in Snowdown, Kent, on Tuesday, April 27.
Mick Duthie, director of operations at Crimestoppers said: "Crimestoppers is here to help people who - for whatever reason - won't or can't speak directly to the police, but want to do the right thing.
"By contacting our charity with information, we guarantee that you will stay 100% anonymous. We're not interested in who you are and will never ask for your details - all we want is information, however small, that might help find those behind this murder.
"Julia's life has mercilessly been taken away. If you know who was involved, please remember our charity's unique service guaranteeing your anonymity is here for you."
Police expanded their search on Thursday with officers seen scouring a mining colliery near to Snowdown.
The move from Crimestoppers comes a day after police released an image of Ms James in the clothes she was last wearing has been released by detectives.
The picture shows Ms James walking her Jack Russell, Toby, while wearing a light blue waterproof coat, blue jeans and dark Wellington-style brown boots.
Police said it was the same clothing, apart from the gloves, which she wore on the day she was killed.
Assistant Chief Constable Tom Richards told journalists the force still has no idea what the motive for her murder was, and no suspect.
He told reporters at Folkestone police station: "I do not know if it's somebody she knew.
"I do not know if it's a stranger attack, of course that possibility is particularly frightening to local residents."