Update 19.5 Dagger Weapons List
• Ceramic Dagger
• Dark Dagger
• Heat Dagger
• Ether Daggers
• Fang Prime
Ceramic, Dark and Heat Daggers are all exclusive Alert Rewards, keep an eye out as they pop up ingame or watch the Xbox Warframe Alerts Twitter feed
The Karyst and Fang Blueprints are available from the marketplace for 20,000. The Fang Prime parts are rewards for various Void missions.
Ether Daggers are a reward from the Stolen Dreams Questline
Sheev is an exclusive Sortie reward.
Lot's of weapons require a Mastery Rank, but by leveling weapons you increase your Rank so this shouldn't be a problem, just make sure to Rank up regularly.Check here for required Mastery Ranks
or the Marketplace.
Prime Weapons require Void completion, often high level Survival/Defense 'Rotation C' drops. These are endgame. I use Primarch Prime Part Guide
I'd suggest keeping an eye out for Alerts and picking up the Stolen Dreams questline which requires you to reach MR3. Then supplementing with the Marketplace blueprints.
.Leveling weapons to Rank 30
The best way to level weapons is to play Survival or Defense missions and kill lots of enemies, then extract for the bonus exp, you'll get lots of weapon mastery whilst unlocking all worlds, doing alerts, unlocking other achievements and just playing the game. Remember to always swap for you next weapon and have one ready to swap.
Lots of Dark Sectors have Affinity bonus (+Shotgun), use a Warframe that supports you but doesn't really kill enemies so you rely on your weapons, being in a group spawns more enemies for more exp.
Elara on Jupiter is a great mission for Affinity, as the anti-moas spawn early and often, and they give a fair amount of affinity around 7-9x the exp of grunts.