I have used WC860 and WC872 as direct substitutes depending on cartridge and bullet weight. Both can still be found as new powder (not pull down).
If using in 50bmg, VV 20N29 is an excellent powder choice as case volume will be exceeded long before even minor pressure issues are realized - even tapping the case, using a drop funnel and/or vibrating the case to maximize volume. VV 24N41 is faster than 20N29 and a better choice for lighter bullets. Both powders are a bit on the pricey side though. Both powders would substitute for H870.
H50bmg is a nice clean burning powder that is slightly faster than H870 (the lot that I have). I am not a fan of US869 in 50bmg or 338LM as I find it to be a dirty powder, although it can be pushed for max velocities in both cartridges.