Is it permissible to seek the help of a soothsayer in order to bear children?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Resorting to soothsayers is prohibited under Sharee'ah and there is a severe threat of punishment for doing this. It was reported that the Prophet, sallallaahu ‘alayhi wa sallam, said that whoever resorts to a soothsayer and believes him, he has indeed disbelieved. [Ahmad]

The woman whose pregnancy has been delayed is required to pursue the necessary means by seeking medical treatment and supplicating Allaah much to grant her righteous offspring.  Not being able to conceive should not lead you to committing prohibitions.  Indeed, there are many women whose pregnancy was delayed for many years then Allaah The Almighty bountifully gave them sons and daughters.

The delay of pregnancy for one or two years should not be a cause for worrying and even sterility should not represent a hardship to the believer, for Allaah The Almighty Says (what means): {To Allaah belongs the dominion of the heavens and the earth; He creates what He wills. He gives to whom He wills female [children] and He gives to whom He wills males. Or He makes them [both] males and females, and He renders whom He wills sterile. Indeed, He is Knowing and Competent.}[Quran 42: 49-50]

Bear in mind that this world is a test and the successful person is the one who is patient and content with the Divine Decree of Allaah The Almighty and His Predestination.