Do you have various sizes of BG and LMB in the pond ?

To feed the LMB you need fish the size they can eat now (3-5in). So any fish you buy to feed them now will be eaten thus the $ spent . Large BG or large tilapia will produce forage for smaller LMB but their yoy ,for now, will not be big enough to feed the big LMB.

If they are that hungry try to see if they will eat pellets. It is worth a try. A mix of sizes of tilapia and some BG would work but some $ worth are going to be eaten.