Once arrive in Cambodia you will learn that shopping a national sport is for locals and tourist as well. Bargaining is roughly a must and mostly there are no fix prices. With patient and a smile you will enjoy this little national game and get a reasonable price. In general you should start with 50% of the sell price and finally you will meet each other somewhere in the middle. By the way, for Jewelry Shopping at anywhere you much be sure by your own as your guide could not value the quality whether they are real or fake things.
Please note that foreigners will pay more than locals. This is not the rip you of but its part of the culture in South East Asia and nobody feels blameworthy.
To meet the real shopping experience you should visit a local market (Phsar) and see or play the game of bargaining to find your souvenirs. Local markets are very busy and the smell can be shocking sometimes, but once again, with a smile you will discover and meet the real bustling Khmer lifestyle.
Beside that, Siem Reap and Phnom Penh are on the international path of shopping. Nowadays there are big shopping malls with fix prices and all kind of modern stores, restaurants and games.
Cambodians famous silk is one of the highlighters to buy. Beside that you will find Angkor statues, Buddha statues in wood/stone; Paintings; Clothes, Hammocks, Khmer scarf (from cotton or silk) and more.