It's probably 54x30 with the two extra inches covered up. The tub installs to the studs so some of the length is hidden behind tile and wallboard. You can definitely find tubs that size.

Do you need the tub maybe just make that space a nice shower unless this is the only tub in the house. Is this just a tub or was it a tub/ shower . Please post some pics of the space and the area yu are asking about. There many 54" ones so if what Susan said is the issue then no problem other than making sure the space you have will work.