Q:

What's the source for the Point Buy alternative ability score rules?

The player's handbook describes the process for determining ability scores via 4d6 drop the lowest (or, actually now that I think about it, it might just be 3d6 and the 4d6 drop might be another variant rule). But I have very rarely actually used that, and instead used the 'point buy' system, starting from 8 and 'buying' points up to a maximum of 18.
I can't find a source for that in the PHB, the DMG or the SRD. Where did it come from? A 3.0 book? A 2e book?

A:

Point buy is described in the DMG
In Dungeons and Dragons, Third Edition, the Dungeon Master's Guide on page 19 on Ability Scores Generation describes Standard Point Buy.
In Dungeons and Dragons 3.5, the Dungeon Master's Guide on page 169 on Ability Scores describes Standard Point Buy.
The SRD deliberately omits the process of creating (and advancing) a character so that folks must buy Player's Handbooks (and maybe Dungeon Master's Guides and Monster Manuals) to play d20 System games.

A:

Point buy first appeared in an official D&D rulebook in AD&D 2nd edition's “2.5e” books, the Player's Option line of supplements. These books tested out all kinds of new ideas, many of which were never revisited, but some of which eventually made it into later D&D editions — grid-based combat and point-buy ability scores being two prominent ones.
There were actually two point-buy methods introduced in Player's Option: Skills & Powers on pages 7–8, as Method VII and Method X. The fact that there are two is odd — the difference between the methods is only a minor note about how to handle Exceptional Strength — but underlines how much these books were experiments in throwing optional rules variations at the player base to see what stuck.
Point buy is one of the ideas that stuck. It would later appear in the 3rd edition DMG, and then the v3.5 PHB.

