I have the Western Digital MyBook 1TB drive. I have installed a 500GB WBFS partition on mine, and a 250GB partition on one of my co-worker's hard drives; exact same model and brand. Both work flawlessly for all games tested so far except for SSX Blur.
http://www.buy.com/prod/western-digital-1t.../205761335.html
This is the cheapest one I can recommend. Buy.com usually delivers within 3 days for this particular item. I think it ships from Pennsylvania, if I'm not mistaken, and $117 is the cheapest I can find.
Both Target and Wal-Mart carry this exact drive, in the price range of $120-$140, and you can check availability in-store, online. I got mine from Target.
I have also successfully tested the Sandisk 8GB Cruzer Micro. It also works flawlessly. Radioshack usually has them for $19.99, or you can order it online from Buy.com from the following link for $20.78 with shipping.
http://www.buy.com/prod/sandisk-cruzer-mic.../208715954.html
Even if you don't have a shit-ton of Wii ISOs, I still recommend the WD MyBook 1TB drive. I have exactly 93 ISOs on my 500GB WBFS partition, and it takes up approximately 220GB. I ripped every single burned DVD that I had to the other remaining 431GB, so it was easier to move them to my WBFS partition. I definitely needed that space to hold the ISOs as I was transferring them to the WBFS partition.
Also, I totally recommend using a Windows GUI for the WBFS partition. It takes way too long to rip games using the Wii, and the point of the USB Loader is to save the Wii's laser. Why screw it up with hours of ripping DVDs?