Where did "yeppers" come from?

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.

My impression is that yeppers is a late stage in a lengthier progression from yep (variant of yes) to yepper (possibly a variant of yessir) to yeppers (a further variant of yepper). Tom Dalzell & Terry Victor, The New Partridge Dictionary of Slang and Unconventional English (2006) has the following entry for yep:

yep yes US, 1891 A variation of 'yes'; the final plosive stresses the affirmative and give it a semi-interjection or exclamatory sense.Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995) suggests that the emergence of yep may be closely related to what it implies is the slightly earlier emergence of nope:

yep affirmation by 1891 Yes; certainly; sure; +YEAH See NOPE ["nope negation by 1888 No {fr no plus an intrusive stop resulting from the closure of the lips, rather than the glottis as is normal}"]

Whether nope actually emerged before yep is not clear, however.

In Elephind searches of various U.S. and Australian newspaper databases, the earliest occurrences of yep, yepper, and yeppers that I could find were from 1882 (for yep), 1983 (for yepper), and 1987 (for yeppers). In each case the earliest match was from the United States. Here are those instances.

From "Seeking Information," in the Vancouver [Washington] Independent (April 27, 1882):

"Born in this country?'

"Mizoorer."

Must follow something?"

"B'ar huntin'."

"O! many bears in these mountains?"

"Good deal of b'ar in the Sary Nevaidys."

"Grizzlies?"

"Grizzlies."

"Like it?"

"Yep."

The speaker is identified as being from Missouri and as having spent considerable time hunting grizzly bears in the Sierra Nevada mountain range of California. It is unclear, however, whether the use of yep is supposed to be typical of Missouri English, upland California English, or neither. An Elephind search for nope for the period from January 1, 1880, to April 30, 1882, turned up more than 2,000 false positives (mostly involving OCR misreadings of the word hope) but no confirmed matches for nope. I would not be surprised if yep and nope arose as twin variants for yes and no sometime in the late 1870s or early 1880s.

From "Commune..." in the San Bernardino [California] Sun (July 17, 1983):

Appearing to be in his late 20s, Jacob is an easy going, likeable man. He has an amusing habit of answering questions "yepper" and "noper."

...

"We're just growing and seeing new things. And as we do, there are things that fall by the wayside. Yepper. That's the way it goes," he said.

And from a Personals ad in the [University Park, Pennsylvania] Daily Collegian (November 30, 1987):

CHRIS HAPPY B—DAY. We finally made it. Yeppers! It's time to party! Enjoy! Love, Spaz.