You may be able to get the look you want at the 18mm end of your 18-55, so give that a try. But three of the four photos you've posted do look like they used a wider lens. Here are three you might consider:
Sigma 10-20mm f4-5.6 (excellent value, good image quality, definitely the value leader for DX wide angles.)
Nikon 10-24 (wider zoom range might help at the long end, and it's a Nikon.)
Sigma 8-16 (widest lens at the wide end, excellent image quality.)
There are a few other options for wide angles, but from what you've said those are the three I'd look at.