Hey, Some advice. If you cannot find someone to trade you. Put your slowpoke with the kings rock in the GTS for trade, But put it up for a zapdos (for example) on Lvl 9 or under. Or a pokemon which is impossible to get, Like machoke lvl 9 or under. or at least something that you know nobody would have.
Then look on the GTS for a pokemon you would give somebody, as in trade with someone, After the trade has finished. take your slowpoke back. And it will evolve the same as if you traded it to someone. its a handy little glitch for HGSS. Hope it helps.