It appears you can get it from this Japanese import/export company:
Towards the bottom of the website, under “Other Goods” there are two
products, one “Black Urushi”, the other "SESHIME URUSHI"Japanese
lacquer (Large 100g / Small 50g) raw lacquer.
It appears you can contact the company via email, and they take
Paypal. I have no idea how much these products cost, how much
shipping will be on a couple of tubes (yikes!), or whether or not
it’s even allowed in the country.
I did read that un-cured (and sometimes cured ) Urushi is to be
handled carefully, as it can cause severe dermatitis; so wear nitrile
or latex gloves at all times when handling it.
Hope this helps,