Open the order. It carries the store, and the store page has everything they currently have on sale.
That route works however long ago it was, and it is the one to use when you cannot remember the name.
The other ways
Search their name, if you remember it. Store names are not unique, so check it is the right one by the items rather than the name alone. Is my store name already taken?
Your username list of follows, if you followed them.
A link you were sent, which still works.
Follow them, once you have found them
This is the actual answer to the question, because it means never asking it again.
Following gets you told when they go live and when they list. For a store you have bought from happily, that is worth more than any amount of browsing — the second purchase from a store you trust is a better buy than a first purchase from a store you do not.
If the store has gone
Closed — the store page says so. A closed store can reopen, and following it still works if it does. How do I close my store?
Quiet — not closed, just not listing. The store page shows what they have had.
Deleted — then it is gone, and your order and its history stay with you regardless. The order, the invoice and the return route do not depend on the store still existing. How refunds work
If you are trying to find a thing rather than a store
Different problem. Search and the category pages are better than hunting for the store you half remember, and the store that had one may not have another. Can I ask a seller to find me something?
The habit worth having
Follow a store the moment an order goes well. Not later — at the moment you open a parcel and it is what you hoped.
That is the only reliable record you will have of a good seller, and it costs one tap at the one moment you are certain.
More: how to find a seller you trust, can I ask a seller to find me something, how to find a show worth watching.