B1 vitamin can easily be found at gardening supplies store. Most of people used it as root stimulant and anti stress. I'm using B1 mostly for new repotted plant, by spraying B1 solution into the media and leaves. Some grower use B1 regularly (once a month) in order to stimulate root growth.
Some web pages stated that B1 for stimulating root is only myth and the others say that B1 need to be combine with root hormone or plant growth regulators in order to achieve maximum result. Which statement is true, I don't know, but many growers still use B1 nowadays. Therefore during B1 application, I mix B1 with Atonik (Plant Growth Regulators)
Links related to B1 and plant growth regulators: