There are lots of good powders for the 155 scenars including VV N150 and N550. I don't know if they quit making 540. I have some in the garage. I had a half decent load with 540 and either 155sc or the 168 smk in my 12 twist rifle, but I moved to a 10 twist rifle and have not used in since. I would say that 540 is not near as popular as 150 and 550.
Why would they discontinue it when according to their data it is the powder that gets the most velocity out of 155gr Scenars? I've always wanted to try it but never have because of the price.