Where did the slang word "yeppers" come from? I have googled it and found dates as far back as the 30s and geographic areas including Florida, the midwest and Pennsylvania.
Yep first showed up in the late 19th century US and spread from there. The Oxford English Dictionary ("yep, adv. and n.") speculates that it comes from "Alteration of yes adv., with an apparently arbitrary element." It's rather like "nope" in that way - the /p/ sound is an arbitrary ending that probably manifested first in oral speech as a quick reply. Its first published uses tend to be in quoted speech:
1883 Xenia (Ohio) Daily Gaz. 31 Mar. ‘Yep,’ said Uncle Rube, shutting one eye and taking a comprehensive glance of the two youths with the other.
1891 Harper's Mag. Nov. 970 He gently and peacefully murmured, ‘Yep’.
1897 R. Kipling Captains Courageous x. 222 ‘Like Lorry Tuck?’ Harvey put in. ‘Yep.’
Yeppers is a diminutive or emphatic form of yep formed with the colloquial suffix -ers. (See preggers.) It isn't usually included in dictionaries, and in corpus searches from the early 20th century the overwhelming result is a misscan or misprint of peppers.
An early usage emerges in 1929, though it appears to be a nonce formation that uses -er to refer to people who say yep (from The Literary Digest, Volume 99, 1928, p. 29):
A Year or two ago I tried to stir up Mr. Meneken to lead a crusade against yeppers and yeahers, but altho he exprest sympathy with my purpose, he did not do anything about it."
As an affirmative, the earliest example I can find is from the magazine Descant (1987), p. 13:
"Yeppers." Captain Andy jumped on the dock again to release the holding ropes from the cleats.
The form may go further back in time; again, this is most common in oral speech or quoted speech, so there most likely won't be one traceable point of origin.