There is ballistic quality shot and reject shot....no need to pay for ballistic shot for slinkys.
There is standard size and magnum size. When slinkys first came around, circa 1985, they were standard size, which is about 20-22 caliber and its associated para cord.

Then magnum para cord became available which allowed .250 - .270 dia shot. I evenually converted to magnum size and abandoned standard size.
I use those plastic key tags with the durable hook that you can get at the office supply stores to organize the various sizes. It gets where you go to a known run and know that it take a "seven shot" magnum slinky. I mark the plastic key tags 3 through about 11 so I can quickly reload if need be.

Now after dragging you through that, slinky (reject) shot is available at any decent sporting good store.

Edit:
We used to use basket ball net cord to get magnum size before the olive drab became available....sometime we had some weird colors