I don't know how many of this group are in the UK, but we have
problems in getting materials. I have used carragheen moss for a bath,
in both powder and as dried sea weed. Specialist Crafts used to sell a
wonderful introduction to marbling set, you got carragheen, alum,
dish, comb, ox gall and colours. I am not quite sure if they are still
sellling it but it was quite good.
I got the dried carraheen from Cornlissen in London.
I have used Pebeo thickner for a bath and it's quite good.
You haven'nt mentioned what to use as a spreading agent, The ox-gall
available in art shops is for water colour painting and is'nt strong
enough. However I understand that a French company has an ox-gall that
is very dark and it works.
Consider Marblease from Liquitex, you can use this with their acrylic
Liquitex do a wonderful leaflet on decorative paint effects, including
how to do marbling with there products.