Of gates demiplane and forks

[Ectar]

So, my party week be facing a level 20 wizard soon. Unbeknownst to the party, the wizard is usually astral projecting from his personal demiplane. When this is discovered, I imagine the party will want to go there. Question is, how?
Planeshift requires a tuned fork, which the party can't have (personal demiplane and all).
Gate doesn't mention requiring a fork, but it does say that it works like planeshift. On the other hand, couldn't the wizard just prevent the gate from opening in his personal demense? Or does a demiplane not count as a planar realm?

Probably the most obvious answer is to have the party loot the fork from a very high ranking subordinate, but that doesn't quell my desire for knowledge on the other bits.

[The Sideromancer]

Demiplanes hang out on the astral, so you may be able to find the coordinates by asking various Astral natives (or incapacitating the wizard and following the cord), and use that info to make an appropriate fork.

[Son of the Veterinarian]

Does the wizard have a minion who has to enter and leave the demiplane? capturing them might be the best way in.