hia mate,
the tank is no problem, i also couldnt find a tank for caustic dipping and hence started making my own tanks!
i can make you a tank purpose built, but will need the dimentiones you are looking to use, also the inlet point and size together with drain point and outlet size.
call me on 01239 621821 and if possible fax me a diagram on 012139 615961
prices range from £240 - £480 + vat and delivery depending on size, of course if you go mad and want to dip a whole arga!!! the price may increase.
regards ben