What's a good source of generic pieces/components?
You have several options if you want to collect a large set of generic components so that you can throw your own prototype together.

Thrift Store games you buy just for the components

If you frequent thrift stores regularly, you can pick up cheap games ($2-$4 usually) specifially just to harvest the components from them. Some good examples from the list above are (with linked pictures of the components):


Scrabble : 100 Wooden Tiles. The backs are blank, or you could have the letters have some meaning within your game, or cover up the letters with stickers.
Risk : Colored Wooden Cubes/Plastic Stars. Old and Classic Editions come with wooden cubes.
Yahtzee Jr. : 1" indented Cubes. Useful if you want to make custom dice.
Advance to Boardwalk : 4 Colors of 20 stackable squares. Upwwords comes with stackable letters.
Trumpet : Cards numbered 1-11 in 6 different suits, when you want to design 5-6 player games and a standard deck won't do.
Bottletops : Hundreds of Wooden disks, most white, some red.
Bingo/Sequence : The $1 Store version of these usually come with red plastic "tiddlywinks", or numbered cardboard chits (useful for wargaming).
Pente : Colored glass beads in two colors.
Monopoly/Global Pursuit - Paper money.
Dicecapades : More dice than you will know what do do with.
Sorry/Pachisi : 4 Colored pawns in 4 colors.


Buy a Piecepack or similar game system

Piecepack is a game system that comes with some predefined generic playing pieces. It is an open board game development platform. Anyone can design and release their own games using Piecepack components, and there will already be an installed player base with the components to play it. The components are offered by different vendors, this one is $30, the components in a standard Piecepack are listed here.

There are other game systems like IceHouse (I am not certain of the openness of this game system. Looney Labs owns a patent on it, but it may have expired), and Orion. For more information, see this page.

Buy generic components from websites.

Print-N-Play productions offers a wide selection of colored wooden bits, meeples, dice, and other game accessories.

Educational Supply stores offer colored wooden/plastic cubes, colored tiles, and other tools that could be used to prototype a board game. I have purchased a tub of 1000 plastic 1 cm. cubes from a local educational supply store for about $22, although prices will vary by the size of the supplier.