Mag extensions aren't an option for a mossberg 500 or Maverick 88, as the barrel screws into the end of the magazine tube. You have to buy a whole other magazine tube and barrel too if you want more rounds.
What is your reason for wanting a larger magazine? If you were looking to get into 3-gun then maybe I can see a real use for a larger magazine, but for clay shooting you only load 1 or 2, hunting is limited to 2 in the mag, and realistically the time it takes to load and fire 10 rounds is the same whether the magazine holds 5 rounds or 10 (Assuming you're single loading, and not using fancy loading techniques or loaders then it takes just as long to load 5, shoot 5, load 5, shoot 5, as it would to load 10, shoot 10.)