Hi chickmchikie,
Good luck with your chicks and the feed situation.
It will be important to provide the feed that chicks need for their optimum growth. I was just looking at the book 'Storey's Guide to Raising Chickens' , and there is a table that states that
for Layers 0-6 weeks their feed should be 18-20% protein,
6-14 weeks 16-18% protein,
14-20 weeks 14-16% protein and from
20 weeks onward 16-18 percent protein.
The feed sacks for chickens at your store should have labels on the bag that give you the precentages of protein. You can see that the first six weeks have the highest requirement and very possibly your chcks can make it that long on the 50 pounds.
You could then buy what your store has available and if needed you could supplement them with some protein like mashed boiled eggs. Storey's Guide even suggests that you can put powdered milk (dry powder) in with their dry food to give a protein boost.
You do have a lot of options on the internet, and it will be expensive to get it shipped, but then sometimes when we travel around, we don't realize that the driving we do ---even to get to the local co-op (we have a feed store in town here, but about 45 minutes drive to our co-op)--the driving we do is an expense.
The difference between the non-starter feeds often is the amount of protein. You may be able to boost the protein that your chickens consume by using some of your table scraps, for example if you have scrambled eggs, and there are left overs chop them up with the feed you have if you end up with something like 'flock raiser' with a protein of 18%.
So you may be in very good shape with what is available in your town.
Meanwhile, you will have a few weeks at the minimum to research your alternatives, and if you make a trip during that time to a bigger city with more feed choices available, you may be able to pick up a sack of feed before your other runs out.
Congratulations on getting your chicks--- tomorrow right?