Anneka Sherratt, 31, was discovered at her Plymouth home on Monday, where it is thought she may have died on Friday.
Her death is not being treated as suspicious and the cause is yet to be established, police said.
Friends have collected more than Â£3,400 towards Ms Sherratt's funeral costs and in aid of her son.
The creator of the fundraising page, Emma Louise Fowler, described Ms Sherratt as a "very kind and friendly" person who "will be sadly missed by so many".
In a message on the page, she said Ms Sherratt's "lovely 3 year old boy...had to defend for himself for 3 days (sic)".
Others paid tribute to her online.
David Elford wrote: "Anneka you were so special to me and I loved you more than words can describe. I will never forget you and I will always think about you and miss you."
Louise Stedham added: "My best friend for 4 years I'm going to miss her so very much we were practically inseparable. Love you so much sweetie."
A spokesman for Devon and Cornwall Police said they were called to a property on Savage Road at 13.15 GMT on Monday.
He added: "A woman in her 30s was found deceased at the property. The death is not being treated as suspicious and a file has been prepared for the coroner."
A spokeswoman from Plymouth Coroners' Court confirmed an investigation was opened into Ms Sherratt's death on 25 February.
"She was found at her home address, the cause of death is unascertained and enquires are continuing, there are no suspicious circumstances," she added.
A three-year-old boy spent three days in a flat with the body of his dead mother before they were found, it is understood.