Just saw this thread and thought I’d help
Just buy “Gordon's® Permethrin-10 Livestock & Premise Spray” ( you can get it at Tractor Supply for around $8 ) and mix it one part Permethrin to six parts distilled water.
There are a couple ways to recharge the pads
1st – you can put it in an eye dropper (visine bottle) or similar drop application type bottle. Put 2 to 4 drops in the center of your used Thermacell pad and let it soak in for ten minutes or so. Recharges your Thermacell pad for about 8 hours of use.
An 8oz bottle from Tractor Supply, at six to one mix ratio and using 4 drops maximum per pad refill, should last you about 1,200-1,400 refills.
2nd - As I'm sure you know, when you use up the effective solution on a new Thermacell pad, it turns white. To recharge this feature, I just use food coloring. (I use yellow, green, even pink ( lol ) instead of blue) What I do is make up a batch in a small jar ( like a baby food jar or similar type ) with 1 tablespoon permethrin, 6 tablespoons distilled water and just a few drops of food color. Then soak your used pads. They will recharge and re-color..

Hope this helps