I managed to get some compost after checking out with friends where they were doing their food shops and if there was any compost left in stores. Aldi, Lidl and Asda had some left near me but limited stock. I got some from Lidl when I did my food shop although their stock was very low, you could only buy three small bags and that was last week. You could try ringing or contacting stores on line and do your food shop accordingly.
We all had horses @GemmaJF but that's so last century now
Its got to be worth a go @GemmaJF although I would just add the grass clippings as wood chips take ages to break down. Manure + chopped greens should only take about 4 months to break down unless the stable bedding is wood chips? Course I could just be talking a load of farmyard waste