Sarah - you sent me back to the books on this one ... since bitter almonds are not available in the US. But, thanks to Harold McGee (On Food and Cooking: The Science and Lore of the Kitchen - Revised, page 506) I think I found an answer for you.
The flavor and aromatic component in almond extract is a chemical called benzaldehyde ... which is found in wild or bitter almonds - something missing from our sweet domesticated almonds. "Pure" almond extract is made from bitter almonds, "Natural" almond flavored extracts contain the benzaldehyde extracted from cassia bark, and "Imitation" extract contains benzaldehyde synthesized from pure chemicals. So, whether it's Pure, Natural or Imitation almond extract - it's going to have about the same flavor.
ALTERNATIVES: I can only think of two. (1) Grind some sweet almonds into an almond "flour" (powder) and add that to your recipe. (2) Make your own extract ... grind some sweet almonds and soak them for a while (several days/weeks/months) in 80-Proof Bourbon or 100-Proof Vodka or 200-Proof Everclear ... bring just to a boil and then remove from the heat and allow to steep covered until cool, then bottle. The alcohol content is 1/2 the "Proof" of the alcohol you use ... 40% for 80-Proof, 50% for 100-Proof, 100% for 200-Proof.
Added 26 March: Sorry I forgot this part ... after allowing the almonds to sit in the alcohol for a week or more (shake every day) ... put into a sauce pan and simmer uncovered until reduced by half. This will cook out some of the alcohol and concentrate the flavor. When cool - strain thru a coffee filter and discard the solids.
"Pure" extracts of anything are generally alcohol soluable ... and will be 20%-40% alcohol. Look at the labels the next time you're in the spice isle.
Hope this helps, Sarah!