Strip back some insulation and measure the OD. On buses they could be 0 all the way to 3/0. My local napa has the lugs on the shelf. You can get a cheap hammer crimper for 20 bucks, Or go more expensive and get the lever style.
In the shop we have a handheld hydraulic crimper that works pretty slick for on-vehicle repairs.
While you're at the store, you can get some heat shrink tubing for it as well. Sealing it up nice helps to keep the corrosion out.
O'Reilly auto parts has the heavy duty shrink tube with the sealant to properly make your battery cables. HST210R for the red and HST210 for the black. Made by standard motor parts. I'm sure those numbers will cross to NAPA (no American parts available) or Carquest / Partsquest. As far as getting a long life out of your cables keeping the corrosion out is the most important thing you can do. I also use NOCO protection spray and pads. Have been using them for decades. Don't use an open flame or other ignition source on the shrink tube near the batteries. 8 D batteries make a hell of a bang when they explode.
Mine were 3/0 negative and 4/0 positive from batteries to starter. The starter bolts they attached to required 1/2 inch holes. Napa had the lugs. I recently upgraded my starter and repaired both battery cables. I have the International T444E.