I know rebuttal witnesses do not have to be designated prior to trial, but I cannot find the case that states that. Any quick help is appreciated!

I hope this Helps
The State is not required to give the defense notice of the witnesses that the State may call in rebuttal.
Hoagland v. State, 494 S.W.2d 186 (Tex. Cr. App. 1973)

There is a more recent case, written by Judge Cochran, that deals with notice of extraneous offenses (and by parallel, the witnesses). Name of defendant was Jaubert. Cochran said notice not required for evidence used in rebuttal.