Hello.
When you say loose, if they are undamaged, just unglued, then simply glue them back!
Most of us have glued the glass into the plastic frames because the sticky foam tape dries up and loses its sticking power. It is a usual recommendation for anyone who hasn't reglued their glass panels to do it before they fall out - especially important on wall-mounted systems!
If it does need replacement glass panels, then theyare, as you say, the same as the 9300, which was the last of these Beocenters, and although I don't think they are a currently listed item, someone may have some new ones in stock somewhere.
Failing that, it's 2nd hand time. Sometimes a non-working (but cosmetically nice) Beocenter will appear on eBay, and would make a good donor.
Hope this helps!