The letters are going to addresses whose waste is sent to Associated Waste Management in Shipley where the baby girl's body was found on 19 May.
The site received 97 loads of waste the day before, 51 from Bradford and the rest from elsewhere in the county.
Police hope an appeal by letter may jog the memory of a potential witness.
The deliveries will be made to addresses in more than 2,000 post code areas, while a second letter will be sent to businesses to appeal to staff for information about the case.
Inquiries have so far revealed the baby was a newly-born white girl who died two or three days before her body was discovered by staff at the Valley Road site.
A West Yorkshire Police spokesman said: "It is also believed the baby did not have a hospital birth and locating the mother to make sure she is safe and well remains a priority for officers."
A team of detectives continues to work on the investigation and police have also been carrying out forensic work to examine other possible ways to identify the baby girl.
Det Supt Sukhbir Singh, who is leading the investigation, said: "The answer to finding the mother of this child continues to lie within the communities of West Yorkshire and we are carrying out this new information seek to see if we can uncover new information to help us find her.
"Any case which involves the death of a baby is harrowing and we are determined to explore all avenues to both identify this baby girl and locate her family."
Police are sending letters to 37,000 homes in West Yorkshire from this week in a bid to find the mother of a baby who was found dead at a waste site.