Where can I find an Amulet of Mara?

You can buy one from Maramal in Riften. He is usually in the Temple of Mara, and sells it for 200 gold. He also happens to be the guy who carries out the marriage ceremony, and explains the whole process before he'll sell one to you.

Short answer

You can get the amulet from Quest, Chests, Merchants, Random Loot and some Places.

Long answer/explanation

1. Quest

The Amulet of Mara can acquired from the quest The Book of Love:

• Quest Giver: Dinya Balu
• Location: Temple of Mara in Riften
• Prerequisite: Speak with Dinya Balu
• Reward: Agent of Mara blessing, Amulet of Mara

2. Chests

• Chests (general)
• Chests in:
  • Mzinchaleft Depths (hidden room underwater)
  • Blackreach (war quarters)
  • Knifepoint Mine (in the shack with Boethia's Champion during the quest Boethia's Calling)
  • Reachwind Eyrie
  • Nchuand-Zel (Control Room)
  • Fort Neugrad (outside)

3. Merchants

• Temple of Mara in Riften: speak with Maramal about the temple's beliefs after you meet him in The Bee and Barb, and he will present you with the option to purchase one from him for 200 Gold
• Fort Dawnguard in Dayspring Canyon: Sorine Jurard may have one for sale

4. Random Loot

• Dwemer cabinets
• Random spawn as dungeon loot
• Dead Bandit (in a hay cart near a fire) in Northwatch Keep
• In one of the two dead Riften Guards in Shor's Watchtower before "meet the men" in the quest Compelling Tribute, as an Imperial
• Also may be found as loot for a dead or alive Whiterun soldier when fighting the first dragon in the main quest at the Western Watchtower
• Rarely, some Guards are wearing it, and it is able to be pickpocketed

5. Places

• Bee and Barb (an inn in the town of Riften and in Riften's Temple Of Mara)
• Stormcloaks General (during the defense of Whiterun)
• In a tent along the coast northeast of Dawnstar (From the entrance of the Dawnstar Sanctuary (facing the door), go left and follow the beachline (you may encounter a few Horkers) until you see a battered tent, the amulet will be inside on the left bedroll. If taking the amulet, then returning after a few days, it might have respawned a new one)
• On the Emperor's ship, the Katariah
• Proudspire Manor in Solitude (after purchasing the upgrades)
• On the boat at Orphan's Tear (in a chest behind the table)
• Next to the word wall on Eldersblood Peak
• Inside the wooded structure inside Labyrinthian
• In one of the upstairs bedrooms during the "Diplomatic Immunity" quest when you're in the Thalmor Embassy
• In the Ansilvund Burial Chambers
• In Radiant Raiment within Solitude
• In Madesi's storage container with Madesi's ring during the mission A Chance Arrangement in Riften

NOTE: Looting and wearing the enchanted versions of the Amulet of Mara found on NPCs will not function like a regular Amulet of Mara until the player speaks to Maramal about marriage.

Source: Amulet of Mara - The Elder Scrolls Wikia

You can buy an Amulet of Mara in Riften in the Bee & Barb, and there will be a priest — talk to him. He sells it for 200 gold.