The evidence for Sul (which may be short for, or latinized into, the Roman Sulis) is the Roman name for Bath "Aquae Sulis", and a variety of objects dedicated to Sulis-Minerva, which can now be found in the collections at the Bath house museum at Bath. The assumption is that Sulis was a local deity that was combined with Minerva by the Romans.
There is actually a spring known as Ffynnon Sul behind Kidwelly Castle*, near Swansea. The oldest recorded name for it is Funnon Syell (1597 C.E.: pronounced Su-ilth?) though it is very far from clear that there is any relationship - firstly as there is ~1500 years between the Sul of Bath and Syell of Kilwelly in which the spring may or may not have had a history, and secondly because subsequent names pick up on Sul being "Sunday" in Welsh and label it "(St.)Sunday's Well" (1624; 1678 C.E.)*. In actual fact, it is much more likely to be from St.Tysilio, who's name was often shortened to "Sulien" (particularly in Brittany*; to/ty being an honorific prefix*), though, rather unusually for a 'celtic' saint, extant material on him doesn't mention any springs popping up around him*.