If you run out of disposable diapers you can use a shirt or towel instead to make a diaper and then use a plastic trash bag with leg holes cut in it to protect the child or baby.
However if you can get more disposable diapers then that would be much better.
The alternatives to disposable diapers are not the best and will usually leak on the baby and toddler and defeat the purpose of the diaper.
If you cannot afford disposable diapers then check with your food bank about a diaper bank as some places have diaper banks that will give you free disposable diapers if you qualify.
Or buy some cloth diapers and use plastic pants along with the cloth diapers which would be better than making a makeshift diaper.
They also make all in one cloth diapers that are even better than regular cloth diapers but both cloth diapers require washing.
To prevent running out of diapers it's best to stock up on them and get the cheaper brands of disposable diapers if you have too.