Which is the most effective Sniper Rifle, and where can I lay my hands on one?

Asking which sniper rifle has the highest DPS is kind of a bad question, IMO. The most important question that you should be asking is, "How easy is it to hit successive headshots".

All the damage in the world isn't going to save you if you can't hit jack squat.

The M98 Widow had the highest DPS of any sniper rifle in the last game, and its probably still one of (if not the) best in this game also. The problem with this from a DPS standpoint is that it is single shot / reload.

The Black Widow (available from Spectre Requisitions) does almost the same amount of damage as a Widow, but it can fire multiple times without reloading, meaning it should do superior damage if you're more accurate. (You can also get multiple damage bonuses from cloak if you're fast enough, something impossible with the widow because of the reload).

You should also check out the Javelin, which is found on Rannoch. It's like the Widow, but focused on shields/barriers instead of just armor, which makes it very good for banshees.

M-98 Widow

Base Damage: 368.3 
Default ammo: 1 / 12 
Found during Priority: Thessia on the ledge where the Asari sniper is. 

Hope it helps

'Best' is of course subjective, 'Highest Damage' will be the Geth Sniper Rifle (called The Javelin) that has the highest damage per hit of any weapon in the game, but a slow attack speed and a 'power up' time when you attack of approximately half a second. This will pretty much always one shot any non-special enemy throughout the entire game with a headshot, including in multi-player on high difficulty settings, but takes some practice to get used to the lead time on the shots.

As I said, 'Best' is subjective but my personal opinion on it is The Javelin.

How to get it:

• Single-player: On the path after a crashed escape pod during Rannoch: Admiral Koris. (Purchasable at Elkoss Combine Arsenal Supplies if missed during mission.)

• Multiplayer: Randomly rewarded by purchasing an item pack.

Points of interest:

• In single-player, the Javelin deals roughly the same damage as the M-98 Widow, however in multiplayer it deals about 20% more damage, thus placing the Javelin as the most powerful single-shot weapon in the game, Claymore aside.

• The Javelin is a very powerful piercing weapon, starting off with its own natural high piercing thickness and high damage when piercing, and when paired with the piercing mod the Javelin can even pierce thin walls to pick off enemies with headshots.

• With the highest-level piercing mod, the Javelin is able to shoot through almost any cover, including thick walls and floors. This is most observable in multiplayer, where the battlefields are on different levels and many different locales are present. In fact, a level V piercing mod allows the Javelin to fire straight through the floor of array control on Dagger and still get a one-shot body kill to standard enemies on silver.

It is also worth noting that this weapon gains bonuses from the 'Networked AI' ability, further increasing it's damage and reducing it's weight and gains additional benefit from the ability as this is a Geth weapon.