How to find Dad's will?

Any property left in your Mom's name (Dad is irrelevant and protesting a will made over 10 years ago will likely go nowhere as it would be near impossible to judge Mom's competency at that time, to say nothing of VERY EXPENSIVE for you lawyer-wise) will be in this new will. A will must be registered for probate in your county. If there is no money, or very little, or the money was already transferred to the other siblings I think you are pretty much out of luck.
You should see a trust and estate or a probate attorney if you have questions and need guidance. One in your area can give you your options.I wish you luck.

I don't think there's such a thing as a "joint Will" -- the law in the US sees every adult as an individual first and foremost.

If your mom was living off her and her husband's assets for 12 years, and depending on her health and lifestyle, it is entirely possible that $800K was spent (especially if she was maintaning a home, a car, or required caregiving either in-home or a facility).

I agree that you need to retain an attorney for her state of residence when she passed.

My MIL never probated my FIL's will. She didn't see the need in part because of her early dementia. Now it is too late to undo the problems that caused. Email the different attorneys in the area that your father's will was created in along with a link to info on your father's death and about you proving your relationship. One might respond with a copy.

As said, if the Will was probated it will be filed at the County seat.

If it read "whats mine is yours" then his assets would go directly to her and may not need to be probated. If Mom has Dementia, she cannot sign any contract or Will. She had to be competent at the time. It has to be witnessed and notarized. If your brother tries to probate this Will he had Mom sign, you can contest it. And brother was correct that after Dad died Mom needed to do a new Will. Our Wills say what yours is mine so if DH passes before me, I will need to makeva new Will for my children to inherit

Was Mom in a facility? With 800k she would be paying privately 10k a month. Thats 120k a year. That would last about 6.5 years. Homecare is expensive. Was one or both of the brothers POA? If so, you and other Siblings have a right to see how her money was spent. If brother/s not willing to show you the figures, then get a lawyer involved.

Most wills (depending on state law) that are joint in nature do not go through the probate process if the surviving spouse gets everything. I would suggest that if you know that lawyer's name that made your dad's original will, you start there. They *might* have a copy. My mother had to hide the will her and my dad made because my brother was always wanting to get access to it to see if he was named and given a specific amount when my dad died. There are some logical and illogical places that people hide wills.

Logical places, Bank safety deposit boxes, law offices, trusted relatives, file cabinets at home.
Illogical places, family bibles, car glove boxes, buried in places, in the attic with old photos, under a mattress.

If you can think of where your mom might hide something that she considered safe you might start there. Good luck. BTW I still have my parent's marriage license. They had to hide that from my sister for decades and it was in a dresser drawer under my mother's "unmentionables".

Dad's will would have had to be probated. Go to the county in which he died and request a copy of the probated will.