I still find Ebay the best place to search for 306/319 parts both new and original. Bonanza is also
a good site for vintage parts. You can find original bobbins alone or mixed in with 306 attachments.
The new bobbins you will find are from china and may have a problem on the bobbin winder
but will fit in the case and sew OK.
If your machine didn't come with the bobbin case then you don't know for sure that it takes
the case with the cutout. It may take the one without the cutout. If you still have trouble
I would also get the one without the cutout. They are cheap on Ebay.
I don't know what an L bobbin is but if you search for 206/306/319 bobbins you will find
the right ones. I would try and find some originals on Ebay to have to at least test your
machine with. Watch out for Pfaff bobbins because they look close but aren't and some
people have them and think they are 306 bobbins.