I find in cases like these is is very helpful to determine why the deficiency is occurring if possible.
For example, if you determined that low K is due to your parent material, then perhaps greensand would be the better choice. Greensand is also used as a water filtration media, so you can usually find it at places that sell filtration media.
If you determine that it is a temporary deficiency due to management issues, then one of the other options may be more appropriate.
You can check your parent material here, assuming your not sitting on top of non-indigenous backfill.