See Gordon's T. S. ELIOT: AN IMPERFECT LIFE, p. 173n. she notes that
Anne Chisholm's biography of Cunard suggests she may have been Fresca
but does not mention an affair.
Carole Seymour-Jones also describes a possible affair in PAINTED SHADOW,
but she too is using Anne Chisholm. So Chisholm seems to have been the
one to discover and discuss this. There is also an odd comment, said by
Seymour-Jones to be in Tom's handwriting in Vivienne's writing-book, in
which he refers to cocaine use and Fresca. In the first mention, there
is a source; this is her reading of Viv's own work. Bonnie Scott, in
her introduction to THE GENDER OF MODERNISM, also mentions this
possibility, and she too seems to be relying on Chisholm.
I think you would find the best material in Chisholm, NANCY CUNARD
(London: Sidgwich & Jackson, 1979). It refers to letters, and you
could find other sources.
There are suggestions of other women Eliot at least spent time with.
Liasons with Mary Hutchinson are apparent in TSE's letters, though
exactly how far it went is not stated.