Bitter almonds are, potentially, very dangerous. Bitter almonds contain
cyanogenic glycosides in seeds, bark and leaves; if eaten in large
quantities, causes convulsions and death. 50-70 bitter almonds cause death
in adults, 7-10 in children. 3 can cause severe poisoning.
The confectionery trade often uses Apricot Kernels as a substitute and they
give excellent results - I've used them to intensify the flavour of
Marzipan in a Stollen I bake most years. They're a lot safer and they give
an excellent flavour of, you guessed, bitter almonds.
If you use "apricot kernels" as a search string, Google returns over 2500
hits so you should be able to sort out a supply.
John