Can you access the Kobo bookstore through your PC? You can purchase books through the Kobo desktop software, and I *think* you can hook up your Kobo and sync it though the computer, but I haven't tried that myself, so I'm not 100% sure that will work.
Most books at the Kobo site give you the option of downloading an Adobe DRM version of the book. You'd need to download
Adobe Digital Editions, register for an account. Then when you plug in your Kobo with that software running, you can register the device. You'll then be able to load the Adobe version of the books you purchase onto your Kobo using the micro USB cable.
Note, that using this method you won't be able to sync bookmarks across different devices, but if you'll only be reading on your Kobo that won't matter.
ETA: Using Adobe Digital Editions, you can also load books your purchase at Sony onto your Kobo, and from many other sites that use ADE DRM.