could you pls tell us how much are you willing to spend ? you get 1TB portable HDDs with power adapters for less, than those without power adapters...like i have WD's My Book Essentials 1TB USB 3.0 HDD with power-adapter which i got for 3400/- locally. online you would get it for ~3.35k, and less after applying discount coupon.
you could also buy internal desktop hdd for lesser (~2.7-2.9k) and use it with a usb-casing (Rs. 250-550) as an external drive. many people use it that way too, but i dont know if its recommended or not, though i have been using three internal laptop-HDDs (one 160GB, second 80GB, and third 500GB) as external/portable HDDs with 2.5" usb-casings for quite sometime without any issues so far !
tell us your budget and i'll search for a suitable match and pass-on the link to you.
i bought the wD essential se passport 1 TB with usb 3.0 2.5 " portable HDD for 3780 from lets buy . 2.5 " hdds dont need the power adapter and usb 3.0 is worth in the long run .... so check that out too .....