Daedalus,
A four ounce bottle will likely last you five years, or longer, unless you have large collected california junipers with deadwood areas the size of barn walls...
Attempting to make this stuff from other sources or from scratch can end in disaster, a nasty, smelly disaster.
Additionally, there are products sold in nurseries that CONTAIN lime sulphur, but they are NOT the stuff you want for bonsai. Usually the nursery stuff has an oil additive that will make a mess of your deadwood. The prepared bonsai solution really has no correllary, as it is technically illegal in the US to use it as a wood preservative. The personnel at the National Arboretum can't use lime sulphur on the national bonsai collection for that reason. They have tried to come up with a substitute, but haven't been real successful.