Hello. I would like to turn this hat hair by bing into an accessory that's hat slider compatible. Since it's a very old cc, I get the error "Index was outside the bounds of the array" when opening it in TSRW. Is it possible to access the file? If so, can someone share the mesh with me?

All you have to do is open the package in S3PE and Export > To file the LOD0 to get the .simgeom of the mesh
You can then use Blender or MeshToolKit to convert it to .obj for use in TSRW, or whatever you prefer from that point on.

In case you don't already know, once you extract LOD0, import to Blender. Remove ALL bones. Then add new one, and type in b__Hat_grip__ . Use those upper and lower case letters. Paint as 100%. You can decimate the other LOD's from LOD0.

Thank you guys for your help! I didn't know I could export the mesh from s3pe; I'll keep that in mind when I run into similar issues. Is there a better way to tell which geom the LOD0 is? I exported it all and guessed after viewing them in milkshape.

In s3pe, the 7th column is file size... however, it's some code, I guess, which I don't know how to read. @CardinalSims or someone else her might know.

You can use Meshing Tool Kit (MTK) to add names in the package, under Package Tools. For that, you can just type in whatever you want. MTK will name the files, and LOD's.

The last way I know, is basically what you did. Usually, the biggest file size in kb is LOD0, although with a hair/ hat, or multi group garment, you may still need to fish a bit.

The GEOMs in that specific CC are named, so they say which one is LOD0 on them already.
But for packages that have unnamed resources, I use MTK's Name Those Files as above. The Extras for S3PE plugin (listed on the main S3PE Simlogical page) also allows you to preview GEOM, including their polycount.