Saying it would be hard to find rollers for a Millermatic 35 is like saying it's hard to find a water pump for a small block chevy. They started making that welder in 1971, and sold it for 25 years. It was the first all in one MIG machine, with the wire feeder and power unit in the same box. They sold a lot of them.
I sold mine when I got my 251, almost two decades ago, so I don't remember how the drive wheels are grooved, but with some wheels you can use different wire with the same groove, changing only one side. It's more about being sized right so it performs right, so check the manual. Manuals are free from Miller, no matter how old your machine is.