Where to buy potash (potassium carbonate)

[MrsSpaceship]

My friend wants it for her fish tank, I've never used the stuff, but I've seen it mentioned in the archives for liquid soap.
I've checked my usual shopping spots (BB, WSP, LC). Does anyone use it and have a supplier they recommend?

 

[cmzaha]

Has she tried tropical fish supply companies. That is where I used to buy it

 

[MrsSpaceship]

Yes, she's called around most places haven't had it, those that do are out of stock.