Is it obligatory to continue trying to purchase a specific phone after performing Istikhara twice and encountering difficulties, or should one look for an alternative, knowing that the desire for it still remains?

A servant seeks guidance from his Lord, trusts in Him, consults experts, and then accepts Allah's decree. Since you have sought Allah's guidance (istikhara), be confident in His excellent management and choice for you, and you will encounter nothing but good. Your previous lack of success in buying the mobile phone does not mean it is not good for you; rather, it might not have been good for you at that particular time. Therefore, there is no impediment to performing istikhara again and endeavoring to purchase it. If you succeed, praise Allah; otherwise, know that the best is in what Allah chooses for you.