Bought mine on eBay for a couple bucks. Lasts forever.
Usually takes just a few grains of it to make a fairly dark solution.
Fissidens is so finicky that I've never left it in any dip for more than a few minutes.
Maybe you could do a dip and then quarantine it in a bucket for a week or so and examine it every day to make sure there are no eggs or snails?
Do you know the dosing amounts or where is the best place to buy this?