How is a lawful marriage to a Muslim woman performed in America if she has no wali (guardian)?

According to the majority of jurists, it is generally permissible for a person to assume both sides of a marriage contract, with some details and differences among them: The Shafi'is limited this to the grandfather only, while the Hanafis permitted it broadly if the person had guardianship from both sides, whether original or by agency, or if he was both principal and guardian. Those who permitted it cited what Al-Bukhari related, in a suspended narration, from Abdur-Rahman ibn Awf, who said to Umm Hakim: "Will you entrust your affair to me?" She replied: "Yes." He said: "Then I have married you." As for those who prohibited it, they argued that guardianship is a condition for the contract, so the one marrying cannot be the one giving in marriage. They also cited that when Al-Mughira proposed to a woman, he ordered a man to marry him to her. The more cautious approach is for the woman to go with the man to an Islamic center to appoint someone to marry her. However, if the woman marries herself with her permission and in the presence of two witnesses, it is permissible because it is a strong opinion, where the man says: "I marry you to myself," and the woman says: "I accept your marriage."