You mean powder. Your weapon need to level up to upgrade to next grade. Leveling up need powders. Tons of it. So the cheaper alternative and fastest way is to pass your unequipped weapon to your alternate character on the same account through the stash. Then play your alternate as it will gain powders when you do quests. Use thise powders to level the unequipped weapon. You dont have to equip the weapon to level it.
After you level it up to legendary, you can pass back to your main account to use. From there, just buy any epic/unique weapon from trademarket and do the same sh it all over again until that weapon is legendary also. Then pass back to main and fuse the 2 legendary together to get a mythical weapon. This mythical weapon will be your endgame weapon.