what's the best way to get weapon patterns?

[young bough]

I'm just looking for the most efficient way to get those red weapon patterns to make some good weapons. Thank you!

[hidden valley]

Well, in the crota's end raid, igniting three lanterns (look on yt for a proper guide) will guarantee you one red border weapon when you complete the raid.

Also, there are minor arcana cards in this season that guarantee a red border from the lectern on helm.