This is an interesting question. GB doesn't have drop rates. It has a drop table, and you get random items from that. Any stage that drops the Stardust Carbonara drops it with equal rate and equal turn efficiency.
Reading the table is somewhat difficult, because each stage has 4 drop tables depending on I assume whether you are revealing the heroine, capturing the heroine, or grinding the stage. I think Iiyama Castle and Okazaki Castle should drop it?
I attached the table so you can figure it out yourself if you're really curious.
All that is mostly irrelevant though, because the optimal farming strategy is the Tower.
There is no way to keep Saboru Nobunaga captive as far as I am aware. Her escape is scripted and IIRC independent from other conditions like capturing other seedbeds.