Everything depends on what cab you are purchasing.
Limited cabs like the Q25 and a few others are simply hard to find parts for period, many don't have aftermarket solutions either. Massively produced cabs like Astros, Blasts, Naomi, and yes even E2s have more parts availability and aftermarket solutions.
The same goes with monitors. Some cabs use monitors that were used for more than one cabinet and are easier to find spares. Have other monitors that can bolt right in. Where some cabs are one offs and the only solution is finding a matching tube or fitting a replacement that isn't a straight bolt in solution.
For example: The easiest part to find on a Q25 is the QSound amp. It was used in multiple cabinets: Big Blue and Impress