How would a character go about to acquire one of these?  Can they be purchased?  And, if so, how much would such a tome cost?  I mean, it is a very rare wondrous item.

That all falls up to the DM.

Can I have 2 of the same tome in one character?

Quote from arielo5 >>

Can I have 2 of the same tome in one character?

 
A character may only benefit from the magic of a particular type of magic tome/manual once. So, while you cannot benefit from two Tomes of Leadership and Influence, you can benefit from a Tome of Leadership and Influence and a Manual of Gainful Exercise, since their magics effect different attributes. Both respective attributes would increase by 2, as would their maximum possible values.

In no place does it say that. You can benefit from it more than once, you either need a second copy, or to wait until the magic comes back in a century to benefit a second time.

So is this a permanent buff?

Yes, even inside an antimagic field, their ability score would be increased by 2 as is their maximum for that score.

So if your charisma is at 20 and if you get this tome you can bump it to 22?

yes

(Old question, I know. Offering my findings for anyone else who comes later) 

The DMG says: 

"But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap."

The PHB says:

"The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap."

Neither of these -specifically- says two identical tomes can't stack, but it appears the D&D Adventurer's League FAQ has chosen to interpret specifically the DMG advice as meaning you can only ever apply one of each of the specific named tomes:

"Permanent Effects. Some items are destroyed when used, but grant permanent effects (e.g. tome of understanding). (...)the Adventurers League staff has interpreted the guidance prescribed in the DMG to mean that you can’t ever benefit from more than one such item—no matter how you come by additional copies."

So at my table, no.  One per each.  At yours... ?

Quote from Chatora >>

How would a character go about to acquire one of these?  Can they be purchased?  And, if so, how much would such a tome cost?  I mean, it is a very rare wondrous item.

 Makeing them is probobly a better way and harder to track

this can help kill tiamat with a single frog. no context will be given.

So can I use it twice after one century?

Quote from MyteDragon >>

this can help kill tiamat with a single frog. no context will be given.

 It's been over a year but I need context

Been two years buddy, We want that context.

Dude this book is literally the tome of charisma. This is a really cool magic item.

Quote from MyteDragon >>

this can help kill tiamat with a single frog. no context will be given.

 Does it have something to do with Polymorph or Suggestion?

3 years, still want that context