If you are on a tight budget here is a kit on ebay. Now I can not guarantee the quality of the parts in this kit because of the price and the fact that they are more than likely cheap Chinese made parts. But for $175 you can get both lower ball joints, both upper control arms, and both inner and outer tie rod ends. Front Control Arm Suspension Kit Set New for Explorer Ranger Pickup Mountaineer | eBay
Like everyone else I recommend using Moog parts as well. That's what I did earlier this year, installed all moog components in the front, lower ball joints, upper control arms and inner and outer tie rod ends. If you look around on the internet you can find all this stuff for real cheap. I think I paid $220 for everything shipped to my door by buying the stuff off ebay.
And don't forget to get an alignment after you replace everything.