What kind of wood is recommended for replacing kitchen cabinet shelving?
You did not say where you are located so direct advice is not possible....but:  Here in the USA many of the "big box" stores will happily cut a plywood panel up into the sizes that you require. Just commit yourself to the purchase of the 4' x 8' panel and they can cut you three or four shelves to the sizes you need. 

These stores have a special setup called a panel saw that makes cutting up panels a piece of cake. Do stress that you have to have close tolerance on the dimensions of each piece so it comes out proper for you. Sometimes the young folks employed at these stores will think that "within a quarter" of an inch is good enough.

Also note that if you go to the shelf at the store and buy the ready cut 2' x 4' pieces be aware that:


Usually four of the 2x4 pre trimmed panels costs more than a whole
4x8 panel.
You will get 3 shelves out of each 2x4 piece instead of 1 with lots of waste.
The large panel may have a nice sized chunk left over that you can
take home for a future DIY project.
There may be reluctance on their part to cut the small panels for
you.