I've heard Heroine Z has the best drop rate but I don't know for sure. But without some insane luck (or so much grinding that you get several million altrium anyway) you won't get 4 more drops.
That said, you really shouldn't need 2 mlb purely blooms to beat lancer alter. Bring a support Okita (unless you have your own) with a mlb purely bloom and a bunch of star of artoria CEs and you should be able to do over 1mil damage in a single turn when you get her NP off.
Here's a link to drop rates courtesy of Reddit.
I should mention however that you really don't need two limit broken purely blooms and you really shouldn't beat yourself over the head grinding for that many. I'd honestly suggest grinding for Artorium and then stacking as many Star of Artorium CEs on your team with a single Purely Bloom on your heavy hitter. For me, it's Okita. I'll include a picture below.
And here's an example of how hard that can push your primary damage dealer into hitting (and this is just turn 7 so NP damage isn't quite maxed out but I couldn't stall any more than I did). With the right buffs, you could balloon that damage or possibly do something similar with a massive crit brave attack.
But hey, take it with a grain of salt, I'm not a theorycrafter or anything.