The case was complicated by 24-year-old Casalotti's half-Thai and half-Italian heritage, which significantly lengthened the odds of a match.
The online Match4Lara campaign garnered support from author JK Rowling, British Premier David Cameron and celebrity Stephen Fry, among other personalities.
On Tuesday, the campaign released a video saying that a "well matching donor has been found."
"It's kind of incredible and amazing to think that this person is 1in 25 million," Casalotti said in the video.
"If all goes according to the plan, I will go to transplant soon."
Casalotti said that the campaign would continue as mix-race donors are still underrepresented in databases across the world.
"We would like to encourage everyone to sign up on the stem cell registers. This simple act can help save someone's life," Casalotti told dpa by email.
Casalotti, a student at university in London, said she was most looking forward to "finishing my degree and carrying on with my life as I was before."
- DPA