What sort of build are you doing? Do you actually need foundations? I didn't, I hired a digger guy to dig out the space then spread a heap of hardcore down. Then my joiner simply built our very solid wooden stables on top (we have a block of 3 very large stables). It works really well and is a really solid building, has withstood a lot of wind as we live on top of a hill in NE Scotland
When OH's brother and her sister's husband put up a new shed at the house down in the SW, they just laid a concrete slab and built the shed on top of that.
I've been looking around for info about laying a concrete slab in the garden, to build a permanent barbecue and pizza oven on it. It looks like you need to find out how deep the frost goes down into the earth during the winter, and dig down a little bit further than that. Lay down hardcore, as MagicMelon wrote, up to the frost depth, then put down a cage of rebar, then pour the cement over that to fill it.
You can get at attachment for an electric drill that you use to vibrate the cement, to get out air bubbles and make sure it completely fills the space around the rebar. One of these might cost you £200 to buy, or you can hire a vibrator from the same place as you'd hire the cement mixer.
I'm sure there are plenty of videos on YouTube of people doing this kind of work.
You don't mention the surface area that your stables would cover...
Be prepared for what might look like huge quantities of material... There are calculators around on the web that will work out for you the volume of the slab and the quantity of sand and cement you need for that volume. Here's one (sorry for it being in Dutch, but it should be easy enough to understand). For a slab 12m by 3m and 15cm thick, you'd need 5760kg of sand, 2520kg of cement, 7200kg of gravel and 1260Litres of water...
You'll need to "lay to fall", ie have a subtle slope to guide the urine run-off to a drain in order to prevent it pooling somewhere inconvenient. Also, there is a grade of concrete that is more resistant to animal waste, someone here will know more.
We did 28 years ago. We got someone with a digger to prepare it. We then put rubble down and some steel mesh and had 5 lorryloads of cement which we levelled ourselves. The mesh means you don't need as much cement. It was hard work and not something we would do now we are 28 years older