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with panna cotta the issue is how deep can you go before you run into issues. Here is my thought keep the depth consistant. Use a corelle or a pie plan or a pyrex. You can always make whole sheets of it in pyrex dishes. Use a ramkin to cut out your shapes. You can also just go squares. When you do this make sure it is very very cold. A quick fix is to head to the store and get a lot of foil potato bakers. Its a half potato shaped foil throw away that are very cheap. These would work fine in a pinch and you can toss them later. Do not make huge batches of the mix. If you are making for 50 I would stick with 3 or 4 batches and just keep working with it that way. When you mix gelatin in huge amounts something gets lost in the work time.
Do not panick if they are not perfect that is what couli/fresh fruit and mint leaves, garnishes are for. Good Luck!

For Panna Cotta and similar desserts, I have used my silicon cupcake/baking liners. They run about 9.99 a dozen at Target....which is pricy...but I have found them to be a good investment. The fun thing is that they come in various shapes...round, square, triangle & heart.

Otherwise....Find your local restaurant/catering supply house. They should have aluminum foil ramekin/souffle cups. Here's an online place that has them...I've ordered stuff from them before....very good service:

http://www.cateringsupplies.com/modules/cart/products.php/nav_id/1/page/1/id/1/name/BAKERYAluminum

You can also find plastic versions which may look nicer.

If your refrigerator is big enough, make batches in a half sheet pan and cut them out with a biscuit or cookie cutter. A standard half sheet pan (18x13x1) can hold a little over 100 fluid ounces and could feasibly make 48-54 2 inch cutouts.

Hope this helps!

Dominic
the zen kitchen