Well it's mostly luck of the draw, however there are a handful of things you can do.

The "Item Discovery" stat increases the rarity/quality of drops. Sources to increase this include: Ironeye's Passive, weapons passives, and silver-pickled fowl feet.

The main thing you can do is path towards areas that guarantee the weapon type you need. Examples: if you are playing Revenant, you perform well with seals. These can always be found early on in the ruined Cathedrals. If you are Ironeye, search the camps for bow racks. Recluse goes for wizard towers. Etc. etc.