it uses the Mossberg 500 mag extensions

Marc, this will be a bit more involved. With the Maverick/Mossberg, you can get longer magazine tubes but the way the tube is attached to the receiver and the position of the barrel retaining lug may require a barrel with the lug farther out to accommodate the longer tube. ---- youtube.com/watch?v=ViZZSdaWvY0&t=112s

Yes the mossberg 500 and the maverick 88 take the same tubes/ extensions.
8 round tube here
midwayusa.com/product/1005529614
----------------- or any mossberg extension will work as well