It seems that you could use a small bottle of ammonia with a resealable cap if you cannot find the ammonia capsules.
The bottle of ammonia (1qt.) would last a long time and costs about $1 and is easy to find in any grocery store.
You could make a nearly spill proof bottle easily.
If you use silica gel for anything else (such as the cat litter crystals or desiccant for keepin stuff dry), soak it in the ammonia solution.
Put the silica gel in a smaller bottle, such as an empty bottle of ibuprofen (you should eventually have one of these around).
Put a cotton ball on top of the silica gel.
Reseal with the bottle's cap.
This way, the bottle does not have to have a liquid tight seal.
When needed, add a wee bit more ammonia water to the silica gel.