What is the ruling on this oath, the text of which is: "By divorce, I will not buy anything for myself," when it was found that the speaker had bought a shawl and sandals for himself?

As long as you swore, thinking that you had not bought anything and did not intend to lie, then there is no breaking of the oath (ḥanith) upon you and no divorce takes place. This is the view of Ibn Taymiyyah and Ibn Baz. It falls under the ruling of a "laghw" (unintended) oath because the swearer did not intend falsehood, but rather said that based on his assumption.

The permissible oath is to swear by Allah the Almighty. As for swearing by divorce, it is among the oaths of the wicked and may lead to undesirable consequences.