There is a very good buy on balsa on EBay but I suppose shipping would be a consideration. If you ever need anything forwarded from here, let know and I’ll be happy to do it. Acrylic for example. Forwarding to Europe is becoming a rather common errand for me.I have only found it on eBay in the UK and a fair bit more expensive than the US.
That's a slight draw-back of the method for us - not everything so readily available over here, whereas the CrOx / FerOx pastes you have, @Somerled appear to be good quality (I have the same ones)
I have not found a good UK source for acrylic blocks for lapping film either so I use glass. Even balsa has been difficult to find. Hobbycraft seem to be the only place that has decent stuff Cass Arts in Birmingham or London sell a good pack. All of the local model shops that used to sell it near me have closed down.