Alan Wright and his wife visited Sissinghurst Castle Garden in Cranbrook in September 2015.
He said a man they spoke to took their photo in the gardens. Mrs Wright, from Kent, died three months later.
A tweet by the National Trust, which runs the site, has been shared more than 600 times. A trust spokesman said it was a "heartbreaking story".
He added: "With luck the [person] who took the photograph at Sissinghurst Castle Garden will see Alan's letter and come forward."
Mr Wright wrote to the trust's magazine for help in tracking down the mystery photographer.
He wrote: "I have realised this would have been the last photo ever taken of her. I would dearly love to obtain a copy."

Borth Animalarium said 32 dragons, which were cold and not moving, were left in a cardboard box on the counter by a friend of a disabled man who could no longer take care of them.
Staff warmed them and fed them, and found homes for eight of the animals.
They are a docile and quiet species, and require a heated tank, light and UV sources.
Animalarium owner Jane Mumbray said: "Many people bred bearded dragons last year as they were selling for Â£80 each in some pet shops. However, there are so many of them about now they do not sell.
"They are probably the best pet lizard species of all. Bearded dragons greet each other by waving a front leg and will often greet their owner in the same way.
"It is not a light commitment to take on a pet lizard and should be taken seriously but they are a very rewarding pet."

Jenny Rathbone said some tenants were charged ??150 to have properties taken off the market while a contract was finalised, while others charged ??65 for cleaning the property.
The Cardiff Central AM called for a ban on the charging of such fees.
Ministers have warned of a risk that banning fees would push up rents.
In Scotland, all charges to private tenants other than rent and deposits have been deemed illegal since 2012.
Campaigners have called for similar rules in other parts of the UK.
In a Senedd debate, Ms Rathbone said: "It should be the landlord paying the letting agency, not the tenant."
She added: "Frankly, the letting agencies are getting away with simply not providing a service in exchange for the fee and I think it's something that needs to be stopped as a matter of urgency."
Assembly lawyers have told Ms Rathbone that Wales had the legal powers to impose a ban, despite the Welsh Government previously saying it did not have the authority.
Communities Secretary Carl Sargeant said agents were legally obliged to advertise fees, and he was "not persuaded" a ban was needed at this time.
"I wouldn't want to see the fees - extortionate fees in some cases - just being transferred to the tenant [in the form of higher rents]," he said.
"I would like to learn from the evidence from the experience in Scotland before deciding whether this measure is necessary," Mr Sargeant added.
Sunday Politics Wales is on BBC One Wales on Sunday 16 October at 1100 BST
A widower is appealing to help find the stranger who took the last photo of him and his wife together before she died.
[NEXT_CONCEPT]
A zoo in Ceredigion is looking for homes for 24 abandoned Australian bearded dragons.
[NEXT_CONCEPT]
Students and other people renting their homes have been victims of "widespread abuse" of fees charged by letting agents, a Labour AM has claimed.