I'm glad she's up and about today - great news
If you have only a couple of sheep it's not worth buying in a 50ml bottle of antibiotics as they quickly go out of date. The same with Calciject. It suits us to have them in stock although we throw out most of the contents once they reach their ubd. From experience, the time you need these meds is on a Saturday night when the vet is closed and will not be open again until Monday. If you don't carry them as stock items, then you will be less likely to use them, to the possible detriment of the sick animal. Reaching into the cupboard is so much easier than getting to the vets surgery. With sheep in particular, speed of treatment can make the difference between them surviving or dying.
With situations such as yours, where a sheep was found having been 'down' for an unknown period in the night, the very first thing it needs is an antibiotic injection to prevent pneumonia - downed sheep are VERY susceptible to this - I would say that you are lucky with yours if she didn't have the antiB.
If you need a single dose of any med from the vet, then you must get it for the specific incident so yes, you must speak to the vet direct. For buying a whole bottle, we get these when we are preparing for lambing and simply ask at the desk. Our vets used to rotate the antiB they used each year, so everyone in our area would be using the same one at any time, but they stopped doing that a few years ago. Some sheep keepers use a lot of antibiotic and others, like us, might only use 2 or 3 doses in a year - or more if we have problems. The downside of antibiotics is the development of resistance, so we must use them responsibly so if your vet doesn't know you he/she may prefer to prescribe as needed.
Calciject is also part of the lambing kit - just be sure you get one suitable for sheep, with the appropriate sheep dose marked on the bottle. When a ewe needs Calciject she needs it immediately so unless your vets is very close or you have a neighbour who carries this in his/her lambing kit and you can borrow some, it could be worth carrying it yourself.