Well, best weapons are a matter of opinion. *The two you have should do you nicely until you get to the upside down castle. *When you get there (or if you're already are) you need to search around for a rare item called the Ring of Arcana (I don't know where it is, but you'll get it eventually). *Once you get that, equip it and go to the Library (again, in the upside down castle). *There's an enemy floating around called a Schmoo. *If you kill it enough you'll get a weapon called a Crissaegrim. *It's the best weapon in the game, no argument.
After that, you need to find an enemy called the Paranthropus (large skeleton carrying a club) and kill them until you get two Rings of Varda. *Lastly, there's an enemy that's about twice as tall as Alucard and spews out a flame attack (there are only two of them and I think they're just called Fire Demons); *if you kill it enough you'll get the second best weapon in the game, the Marsil.
There are a lot of other useful weapons; the Shield Rod is fun, if you get two the Heaven Swords have an interesting effect, and the Alucard Sword has a nice effect. *Still, the Crissaegrim and the Marsil are the two best.
In the first castle just stick with what you have until you can either buy better or find a new one. *Don't forget to explore everything. Get a 100% map coverage in every room and keep the Fairy on to help find hidden areas.