You'll spot Cambodian spices and curry mixes packaged prettily for tourists, like the Khmer Curry spice mixes above at the stalls lining the Western edge of Old Market (side closest to Pub Street). Spices are also sold in handmade packaging at gift shops like Senteurs d'Angkor, which specializes in all things fragrant, from spices and Kampot pepper to soaps and incense. The spice mixes above won't be useful for serious cooks, who will want to buy individually packaged spices in larger quantities. For those, head to the stalls in the centre of Old Market or to Psar Leu, another local market on National Highway 6 (one of my favourites) or Angkor Market (Siem Reap's best supermarket).
These, however, are perfect for those of you who have done a cooking class and would like to try making what you learnt at home, but probably can't see yourself cooking Cambodian food on a regular basis. A pre-mixed spice pack means the dish probably won't taste exactly as it did in your cooking class or favorite Siem Reap restaurant, however, it will save you carrying home a range of bulky spices and if all you want is an edible reminder of your travels, they'll do the trick.
Try to buy vacuum packed spices, so they last longer; like anything, fresh is best when it comes to spices. It might also be the only way you'll be able to get them through Customs in your home country. Some countries won't allow them in at all, so check before packing and risking a fine, or declare them if uncertain.