What repeaters? For a guy who likes to hold himself as an expert, your lack of knowledge is really staggering.
92% of Buford's companies reported their ordnance on June 30, 1863. Among those 92%, there was not a single Spencer or Henry. Not one. Roughly 60% carried Sharps. The others carried other single shot breech loading carbines such as the Gallagher, Merrill, Smith or Burnside. Buford himself probably had a Henry rifle--the only one in the entire command. I have held John Buford's personal Henry rifle, so I know that it exists.
The only Spencers in the entire Army of the Potomac were carried by all of the 5th Michigan Cavalry and four companies of the 6th Michigan Cavalry of the Michigan Cavalry Brigade. Those were Spencer rifles--the Spencer carbine didn't go into mass production until September 1863. And on July 1, the MCB was nowhere near Gettysburg--it was at least 40 miles away.
Below is a table that I compiled from the original source documents--the June 30, 1863 Ordnance Returns of the First Cavalry Division, Cavalry Corps, Army of the Potomac--when I was writing my book on Buford at Gettysburg:
Unit | Sharps | Burnside | Smith | Gallagher | Merrill |
8th Illinois | 311 | | | | |
12th Illinois | | 86 | | | |
3rd Indiana | 12 | | | 182 | |
8th NY | 210 | | | | |
6th NY | 232 | | | | |
9th NY | 381 | | 1 | | |
17th PA | | | 127 | | 108 |
3rd WV | | | | 89 | |
I might also point out that Buford and his two brigades didn't just "show up." On June 29, they were expressly ordered to go to--and hold--Gettysburg. They did so on June 30. There was no luck about it. They were there pursuant to a designed plan to have a cavalry screen well in advance of the Left Wing of the Army of the Potomac. Buford had the largest division, and he was trusted by Reynolds, the commander of the Left Wing, so the First Division was the logical choice to do so. It was by design.