"I have a customer who is looking for a 750-1000 gal tank that can be buried and will maintain 40+ psi . Well/pump/booster system."
That kind of thing is not an off-the-shelf item. It would probably have to be fabricated. Another solution would be to find a used one from a chemical plant or an old water system. You might find that sort of thing if you search for "Pressure vessel" in Google.
You would want an inspection and engineering analysis. That size tank is just on the margin of where they usually put a manhole in them to allow inspection.
You may need an ASME coded vessel in your jurisdiction, which will be more expensive. You will need a relief valve if the pumps are capable of exceeding the design operating pressure of the tank.
I would do a serious analysis of why it has to be buried. It may take more to survive possible traffic over it than to withstand the pressure. I would also do a cost analysis to compare with multiple bladder tanks. If it absolutely must be underground, I would consider an accessible vault rather than direct burial.
In many jurisdictions it is not permitted to put hydropneumatic tanks underground, or use them for fire protection purposes. You may want an engineering analysis of the whole system to find the most economical way to meet requirements.