Seymour was five, so he didn’t actually remember all that much; if you want more, the anthology We Saw Lincoln Shot has 100 eyewitness accounts.
Brown University also has a nice historical archive featuring assassination marginalia. My favorite, in the nothing-new-under-the-sun sense, is the Lincoln Assassination Commemorative Three-Dollar Coin:
No labor and skill has been spared to make this Coin Piece the Champion of all medals in this country…. bearing on one side an accurate Medallion Head of our martyred President, moulded from life; and on the other is a miniature Monument, towering high above the rays of the setting sun, each ray poising a brilliant star; the stars, 36 in number, corresponding to the States of the Union, who sent up their wail to heaven, in prayers for the mighty liberator, who we are to imagine sleeps at the base, where may be seen the inscription—ABRAHAM LINCOLN, Martyr for Liberty.
"[S]tamped and milled from a solid Oreide Gold Composite [i.e. a tin-zinc-copper alloy] plate, a fine metal nearly resembling fine gold," it was a mere four dollars.