Alzheimer's is a brain disease, as you know. In order to help someone with this disease you need to understand how it affects the brain. One of the first parts
of the brain that is affected is the ability to see the consequences of behavior. Most of us can 'stand outside' ourselves and see how something we might do will affect us or others; we can predict the outcome of our actions. This ability is lost fairly early in the disease process.
This means that your mother cannot see the problem with spending money for things she wants, when she wants them. She cannot predict the consequence of spending all her money. Because part of the disease also impacts the ability to understand logic, you can explain the facts of money in the account and money spent, and she will not be able to understand.
The best way to handle this type of situation is to put limits on her ability to spend. It is unfair to ask the assistants in the thrift shop to put limits on your mother; that is not part of their job. Give your mother no cash, and she will not spend it.
Remember that your mother's short term memory is impaired. She will not recall exactly how much you have given her, and she can't recall how much she has spent. Do not say no to her, because it is not kind to deny things to an Alzheimer's patient, who cannot understand why the desired item is being denied. Instead, tell her that you gave her money yesterday, or that you forgot your wallet, or that you are running short of cash, or whatever. This is known as a "therapeutic lie". Technically it is an untruth but it is being said not to harm but to protect.
Your mother is not responsible for what is happening to her. Neither are you. Of the two of you, you are the only one who has the ability to protect. It is better for her to think that you are foolish or forgetful than for her to run out of money. Let her blame you for not satisfying her desire to shop; you are the responsible party now.
If possible, take her to a thrift shop somewhere else, or a Dollar Store, or a candy store, or some other inexpensive shop. Give her a small amount of cash, in single bills and coins, so that it looks like a lot. She probably cannot count money now. Let her spend it all and have a good time.