We (4 of us) are going to be in Fussen from the afternoon of Friday July 18 to the morning of Monday July 21. From what I can see both N castle is sold out for our time. However, there is also the Hohes Schloss Castle and the museum. But, I was unable to find how to purchase tickets. Any help would be appreciated. Thank you.
Assuming you are referring to Neuschwanstein and Hohenschwangau, you could start with this website: https://www.hohenschwangau.de/#
Added note: you have posted in the Switzerland forum. The palaces you are referring to are in Germany.
Thank you -- will switch to Germany.