Where are some good locations for traders?
If you have the Vanila game I would recommend Rivet City Traders, they usually have .44 amo for sale but not 100% of the time and not a high number, so if you need to stock up make regular trips. Also an observation I've made is that appears at lower levels the likelihood of a trader having .44 amo for sale seems lower than at higher levels or later in the game.
If you have completed the quest for Abraham Washington Sydney becomes a trader in Underworld and she will respawn .44 amo regularly. Chief Gustav usually had some .44 for sale at TenPenny Tower as well for me.
If you have DLC's then the Pitt eventually allows access to an amo press and point lookout traders I found were very good for .44 amo and also repairs.....