On every single or fix conversion I try the lockrings don't fit. I get the lockrings that come with the kit. The lockring I removed from the cassette was a suntour. Does that mean I have to use a suntour lockring with the cog? If so, where can I get a suntour lockring? The lockrings that come with the kit are either too small or have a different thread.
We need to understand a bit better what exactly you are trying to do.
For a "fix gear" you need a hub made specifically for that (usually). The lockring that comes with the cog should fit. I believe there is only one threading for a fix cog lockring. But that lockring is completely different than what comes on a cassette lockring.
If you're converting a cassette multi-speed hub to a single speed using a conversion kit, I think you should be able to use either the lockring from the original cassette or from the kit. There may be different threadings, so whatever fits your hub is the one to use.
Are you sure you have a suntour "cassette" and not a freewheel? There was a suntour version of the cassette system. I thought that on theirs there was not a lockring per-se but the last cog unscrewed off the freehub body. But I may be wrong about that.
fix, single speed, freewheel, cassette, lockring, etc. are all specific terms with different meanings. I don't know what you're familiar with and what you're not so I won't go into detailed descriptions. But it's hard to help without knowing exactly what you're describing. None of this stuff is very complicated, but hard to communicate without using the right terms.
Ride hard or ride home alone!