Crutchfield sells the 6x8 speakers. You can go on there and input your trucks make, model, ect and it will list all the speakers that fit. I went with Boston Acoustics 6X8 in all four doors. I think all 4 were like $140 and they sound great. There are plenty of good speakers to choose from on there depending what your budget is.
2008 Super Crew 4X4 5.4 Lariat. Roush CAI, Borla SI/SO XS, Gryphon programmer, 5000K HIDís, 2.5" HBS leveling kit, Nitto Tarra Grapplers, Saleen grill with PTM trim, Boston Acoustics speakers. 4 6" 55 watt KC daylighters on Randy Ellis light bar. And I like black wheels on F150's!