Mew is unobtainable in any of the pokemon games unless you cheat, take advantage of glitches( Red, Blue, and Yellow only), or attend an official pokemon event, which lets you download Mew.
You obtain Spiritomb in Diamond and Pearl by first finding the key item, the "Odd Keystone". Unfortunatly, I can't remember where you can find them... If you don't have one already, go into the underground and dig into the walls to find one, although they're a bit rare.
Once you've obtained an Odd Keystone, you need to talk to 32 people in the Underground - however, the hikers don't count, you need to talk to actual people on wi-fi, so you'll need a friend. You can get around this by entering the underground, talking to the person, exitting, and repeating 32 times. Each time you enter and exit, it counts as talking to one more person.
Once you've done this, you need to take the Odd Keystone to route 209, and find a pile of rubble at the lower right side of the route, directly below Solaceon town. Walk up to the pile of rubble and click A on it to insert the Odd Keystone, and if you've talked to people 32 times in the underground, Spiritomb will appear. I'm pretty sure it's at a low level, probably somewhere between 10 and 30, so bring one strong pokemon, and a few pokemon that get progressively weaker to catch it. And don't forget to save before inserting the Odd Keystone!
Aside from trading, there's no other way to obtain a Spiritomb. Luckily, if you've fought the Champion who has a Spiritomb, you may be able trade for one on GTS. Annoying to get it, no?
EDIT: I just remembered where to find the Keystone! You have to go to Hearthome City and go left, entering the route east of it. Go straight down and to the left, and you'll see a Black Belt dude. Talk to him, and he'll give you the Odd Keystone.