Could you fit in a small compost bin, like the plastic one I have in my very small terraced house garden? If so, & you put in kitchen waste such as vegetable trimmings, & any weeds & other stuff you get from the garden, you should have something usable by next year. I put everything in mine, including ivy, orange peel. dead roses, etc. & it all breaks down to good stuff within a year. Leave out the woody stuff & you could get compost fit for most purposes (not seed beds) much earlier. You'd still need to get some stuff in, though, if you want to get started soon.
Stables can be a good source of manure, but not usually well-rotted. There's a risk of a stink. Another possibility is a local gardening society. My local one (East Reading Horticultural Society) is about the cheapest source of bagged-up, well-rotted manure I know round here. One has to be a member, but that's a mere £3 a year. I recovered it in seeds, canes, fertiliser, etc. in no time. Like many others, it has a trading shed (theirs is a breeze-block garage-sized structure) on an allotment site. Web presence may be limited.