Like the rubber troughs, this might be another easy and quick solution.

We use galvanized pipe to carry our water, coat nails, on corrugated metal for sheds and buildings, and even all of your roof vents for the house. Galvanizing is simply coating a metal, like iron, with Zinc. This can be done by electroplating or by a hot dip method in which the part or container (Here's the process) is dipped into molten zinc. Zinc doesn't rust (as most people see brown crud as rust) but it does eventually wear away and the underlying metal will then be exposed to humidity and corrosion.
 
As a result of cheap (and I use this word specifically) products from China, Taiwan, and other Asian countries we've seen the galvanizing industry nearly die here in the US as it 's moved overseas where waste regulations and disposal are lax and labor is inexpensive. Most products you'll purchase will be plated and the thickness of the plating will make an enormous difference in product life: but there's no way to know because this isn't something placed on the label.

I'm not a fan of overseas products or the plating industry in general (Having worked in it some years back), but the low price of galvanized products just might be the thing for you so don't let my minor ranting bother you all that much. My only reasonable gripe about using galvanized materials is that any metal will dent, ding, and bend. Treated roughly in this way will quickly expose rough edges and rust all that much more quickly, possibly exposing your animals to tetanus or just being hurt. If you pursue using galvanized just be mentally prepared and try not to take it out on the sheep if they abuse it.

         Molten Zinc for Hot Dip Process

Cold Galvanized Spray

I've toured the isles of a few feed stores, both large and small, and there are so many metal buckets, bins, and trays that the selection might make you believe that there wasn't anything else to choose from EXCEPT galvanized. Whether you have one, two are several dozen or hundred sheep I'm sure you'll find what you need. The pictures above are linked to a site specializing in buckets and where I got the pictures.

You can even find galvanized tanks specifically for the size of your sheep at some local hardware stores like the one pictured.

Also, I took the time to search the net for a few different ideas and choices so you won't need to work as hard on your search. I hope this starts you off well enough.

Take a look

Here,         (Tank like the one pictured)

Here,         (Bunks, troughs, and bins)

Here,         (Mostly Bunks)

Here,         (Enormous troughs, portable too!)

and

Here.         (80-700 gal round tubs)