You are correct that there are no extant sources that corroborate Plato's claim to have been present at Socrates' trial. However, we also have nothing that contradicts it.
As others have already mentioned in the comments, our two main sources of information for Socrates' later life are Plato and Xenophon. Both were disciples of Socrates, although neither mentions the other.
Other significant sources for details about the life of Socrates are Aristophanes and Aristotle (although Aristotle was not a contemporary of Socrates). In addition, we have some surviving fragmentary works by Aeschines, Antisthenes, Euclid of Megara, Phaedo of Elis and Timon of Phlius. Sadly, none of these explicitly state whether Plato was present at the trial or not.
Of course, it is possible that there were further contemporary sources that do not survive, but in the absence of any evidence to the contrary I think most researchers accept that Plato was present at the trial. This is a particular aspect of the wider "Socratic Problem", and like many aspects of the historical Socrates corroborative evidence is hard to come by.
The Stanford Encyclopedia of Philosophy contains an excellent article by
Debra Nails on Socrates which includes an examination of the main sources for Socrates' life and also of his trial and execution.