You can find an Abra in the White Forest (white 1 only) when you have some people there, other than that Alakazam requires a migration for a previous gen. the best way is to obtain Alakazam, is if you have 2 ds's and diamond/pear/platinum and HG/SS, abra and kadara appears both gen IV sets, you can trade and migrate to your gen 5 game. Note that certain people in White Forest bring certain Pokemon.
Spiritomb only appears when you have the Odd Keystone in to the crumbling tower on route 209. It is obtained by a NPC after you defeat him. Place it on the crumbling tower nearby the NPC. Once you place it you will have to meet 10-30 or so people in the underground tunnels, then you should be able to face Spiritomb there at the crumbling tower. There is one other way that is with the pokewalker and requires a boatload of steps to encounter Spiritomb.