As in working on a "professional" yard, rather than trousers you wear around while mucking out your own horses?
When I was working on a yard, I wore smart ish combat type trousers - the fitted type, rather than the baggy ones. The main reason for this as they usually had extra pockets for carrying around keys, penknife etc etc. You can usually find these type of trousers, of nicely fitted jean type trousers in the likes of M&S, Matalan etc.
Mucking out my own horses, mainly consists of a uniform of tracky bums, and hoodies
You could try the outdoor shops and catalogues for smartish but practical trousers. Cotswold outdoor have a good range.
Get something in a smart neutral colour and wear it with paddock boots/dubarrys or whatever.
Often they have extra pockets, a bit of stretch for movement and also climate appropriate features such as fleece linings for winter or mesh vents for summer, summer weight trousers are often UV protective and fly repellent too - bonus!
Check out brands like craghopper, north face, berghaus, rohan etc