We use (and would recommend a call to....)
Palm House Nurseries (01483 810066)
Surrey GU3 2DF
They carry a reasonable stock of 'indoor plants', troughs and planters made of synthetic and ali troughs and planters and can obtain Stainless Steel etc to order. They also stock all supplies needed etc (I know as we just used them.
Have a look at:
They do a range of large troughs in the materials and sizes you're after.