Is it permissible to perform Istikhara online, and can you guide us to authentic websites for it or create a dedicated website?

The الاستخارة (guidance-seeking) prayer is recommended for permissible matters, and not for obligations, recommended acts, prohibitions, or disliked acts. Visiting beneficial websites, such as Islamic and scientific websites, is a recommended act and does not require الاستخارة. Conversely, it is forbidden to visit prohibited websites, and that does not require الاستخارة either. There are many good Islamic websites such as IslamToday.net and IslamOnline.net.