Your's are probably rivetted on. Pull the knife arm, grind off the rivet head, punch out the rivet body, replace with new knife using fasteners provided. Save the old knife, It can usually be sharpened and put back into service.
Knife kits from any parts dealer. Look around. I'd guess that cutoff knives are universal amongst just about all makes and models of balers. While you are putting it back together, make sure the wiper arm scrapes the heel of the bill hook. Otherwise you are facing knotter problems. You bend the wiper arm to get this contact.