Well in these commercial plant products they use thiamine hydrochloride as far as I have seen, basically what is commonly sold as B1 for human supplementation.
There are various other fancy ones for human like Thiamine Tetrahydrofurfuryl Disulfide, which I have tried a couple of times.
You may find thiamine mononitrate for horses.
Animed brand has Thiamine HCL (hydrochloride) powder which you can mix in water and try to achieve a 0.1% solution.
