As the comment below mentioned Exotic missions and Xur are the way to obtain them. A (brief) list of the mission drops is below: Pressage: season 17 gear Vox: season 16 gear Seraph: season 19 gear Avalon: season 20 gear Star-crossed: season 23 gear Seasons 18, 21 and 22 did not have exotic missions (technically 21 did but it was hidden in the seasonal activity and was only doable through the activity) For all other gear (including the seasons mentioned above) you will either need to wait for Xur to specifically sell them (roughly 1 every 10 or so purchaces) or you can obtain them from his weapon engram as that contains all of the past season legendaries (as well as old world drops but not all have patterns) You can also use deepsight harmonizers to aquire 1 pattern per harmonizer from a weapon you have not aquired the pattern from. Harmonizers are aquired through the season pass.

For Craftable weapons that are no longer available through their original source it is possible to receive Red Borders by purchasing them through Xur when they come up in his rotation, however I cannot speak to how likely this is as I personally have never had to use this method to unlock weapon patterns. In addition to this, certain Activities from the Exotic Weapon Mission rotator will award Red Border drops tied to the Season in which they were released (eg. Season of Wish weapons from the [b]Star-Crossed[/b] Mission). (΄◉◞౪◟◉｀)