All you do is go to the thieves guild talk to Tonilia and buy lockpicks for 1g a piece, save and then attack her till she is on her knees, reload the save then all the lockpicks will be there again

You don't actually need to attack that much, just a punch in the face will do (and it works for other merchants, too, and it refreshes their cash as well). It does make me feel a bit like my character is Patrick Bateman, though, so I rarely do it any more, I usually just add gold or the item I'm after to the merchant's inventory using the console.

Alternatively, if you play on the PC, you can use a console command:

player.additem 0000000a x

where, x = number of lockpicks to add to your inventory

Or you could just make sure you check all containers and bodies you encounter and you wont run out of them in the first place. Well, maybe if you really svck at the mini-game you might

If you kill a vendor NPC, save and then reload their item will reset.

This is happen to all vendor not just Tonilla.

Have you seen my new business cards? The color is Bone.

I've never had a problem with running out of lockpicks. At one point I would buy them from vendors (like 50 levels ago), but now I don't even do that and I have something like 340 stored up.