The Baths of Titus have long been destroyed. They stood over part of the remains of Nero’s Golden House, itself filled with frescos.
A volume published in 1786 and now online, Ponce, Description des bains de Titus, here, contains a general view of the Baths, as they then stood, together with the entrance to the underground areas; plus two maps.
First the overview, including one of the massive exedras:
Next, a map of the underground areas, indicating the foundations of the exedras.
Finally a map of the overground area, with some elevations. I wonder, from the notes on it, how much of this was still standing in 1786, tho. I’m guessing this is merely a reconstruction from Piranesi, etc.
Update: Ste. Trombetti writes to say that in fact Ponce is a pixel-for-pixel reprint of Ludovico Mirri’s Vestigia delle terme di Tito e loro interne pitture, 1776, accessible at the Heidelberg Digital library here. (And, pleasingly, with a nice big “Download” button on it!)