Wiggle room for tire sizes?

So I don’t know much about anything but have this 1982 Yamaha XS 400 that has all stock parts. I’m giving it a pretty basic brat makeover but need new tires (old ones are worn and cracking) so I’m looking for new ones.My question is how much wiggle room is there for tire sizes? The listed sizes are:Front: 3.00-18 Back: 3.75-18The issue is I’m having troubles finding tires that fit, specifically styles like firestone deluxe champions or Shinko super classic tires.Any help would be huge! via /r/CafeRacers https://www.reddit.com/r/CafeRacers/comments/abpqix/wiggle_room_for_tire_sizes/?utm_source=ifttt