anything but old baths, please! the edges are really sharp and horses can lacerate themselves on them so fast.
fwiw i used an old bath for soaking haynets for a while, years ago. tied a 5 yr old up near it for 1 min while i went to get some shampoo, came back and he'd slashed his leg open on the edge of the bath, had to get the vet to stitch him up. really taught me a lesson.
the biggest tubtrugs, or plastic dustbins, or plastic troughs direct from supplier (that's where i got mine, much cheaper than through middleman).
I am sure baths would be cheap, and we could make them safe - but i can't see the landowner being thrilled with this solution! Back to the drawing board
i bought some from a farm shop that were a bargin, at the other agricultural shop a few doors down that has an equestrian section, twice the price!
Otherwise as others have said, dustbins, bath if you could make it safe, or what about a plastic water tank? i saw them in B&Q the other week and thought mmmmm lol!
I have some 75 litre heavy duty plastic garden bucket type thingies, with useful handle for tying to fence post. Got mine for $6 each at Home Depot, that is B&Q to you in the UK.