Tamzyn Rollinson, 29, from Hove, was desperate to buy the same baby blanket she wrapped her daughter in when she was born for a memory box.
But when she looked online, it said the blanket was currently out of stock.
Rollinson decided to private message Tesco to see if they could sort it out and was overwhelmed when they went above and beyond to help her out.
"Hi Tesco, I just want to say a massive thank you to your lovely customer service team," Rollinson wrote on Facebook, shared on the Love What Matters page.
"I sent them a private message last night to see if they could try and find a baby blanket for me that was currently out of stock everywhere.
"I really wanted to get hold of this certain blanket as my little girl was stillborn nearly three weeks ago and this is the same blanket I wrapped her in when she was born so I wanted one for myself to go in her memory box.
"Tesco employees managed to track one down for me this morning and the lovely staff at the Shoreham-By-Sea branch went out of their way to hand deliver me not only the beautiful blanket but a gorgeous bunch of flowers and a card too!
"It was such a kind thing for you all to do and I thank you so much."
The post, shared on 26 June, received more than 1,000 shares and 22,000 likes in 12 hours.
Rollinson received messages of support from other mums who had been through the same thing.
"I am so very sorry for your loss," one mother wrote. "My first daughter was stillborn almost 10 years ago, and I feel your pain so deeply.
"That blanket, and their gesture, will remain in a special place in your heart."
Another mother wrote: "My son would be 25 next month. You will always miss your baby, but know that life has to go on and you have to be strong.
"My memory box only comes out on his birthday now. I can still smell him in his blanket. You have my condolences. God bless you."
For information or support on stillbirth:
Sands: Sands, the national charity, supports anyone who has been affected by the death of a baby before, during or shortly after birth. Call their helpline on 020 7436 5881.