It is a common misconception that an iPad (or iPhone) can remain fully functional with damaged or defective physical buttons.
Whilst Accessibility features (such as assistive touch) can be used to augment the User Interface, these features cannot be used as a substitute for physical button sequences that must be used to trigger system recovery actions. Sooner or later you will encounter a situation from which you cannot recover.
You can arrange repair of your iPad either by (a) visiting the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - or (b) using Apple's mail-in repair service:
Genius Bar Reservation and Apple Support Options - Apple
iPad Repair & Service - Apple Support
For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. Unless you have benefit of an active AppleCare plan, an out-of-warranty repair while expensive will be somewhat less than the cost of a replacement iPad.
If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.