Most (pouch-shaped) Li-Ion batteries just look like square shapes packaged in heavy aluminum foil, with some Kapton tape to keep a small PCB with protection circuitry in place. Any determined hobbyist could buy smaller batteries and the packaging materials off AliExpress to make something that looks visually similar but has lots of space left over for explosives.
With cylindrical batteries it's a bit harder, but ultimately they are just a cylinder with pressed-on end caps. You can disassemble them (lots of videos on youtube), change the contents and reassemble them.
It is pretty high effort compared to just sticking the explosives next to the pager's electronics, but I don't think the barrier to entry is actually that high