Lightbox Tubes

I have a Hancocks Studio A3 lightbox which still works fine but one day will need new tubes.
I recently tried to get spares for the original Philips TLD 15w/950 fluorescent tubes but no joy. Philips can't be bothered to reply and anyone else I've contacted says there is no modern equivalent.
Any suggestions?

Just in case it helps anyone in future, I found a source for the tubes at www.techlamps.co.uk
I suspect they would have other obscure stuff too.