Is there a way to purchase Cydia apps from a regular browser and not from inside Cydia?
Yes, there is. The home page
I don't think you can purchase from Cydia on any website, but there are websites which allow you to view the packages available from Cycia, such as Cydia Search. You can use these sites to determine what you want to download on your computer so that you only have to do the actual downloads from your iPhone. If you look enough, you can find some applications that you can download using your computer, but they do not come through Cydia.
Since the Cydia source is hosted and not actually in-app, @luca590's link is most appropriate right now for access to the store.
Otherwise, when Lima releases, it'll be a different ballgame.