The web is full of gallery websites for CSS design. e.g:
Is there such a thing for startups?
I've also used...
http://www.startupnorth.ca/ (if you're Canadian)
Depending on the type of startup, there's likely niche related sites.
I suggest doing a google search for your competitor, look who's linking to them, and submit to the same sites. A search would look like:
We recently launched, & would be more than happy to feature your startup on http://99Launch.es.