For those in the Tokyo area, there are usually several smaller events happening every day of the year. Finding the events can be tough though and many of them are theme events, like *insert name of show here* characters only, or JUMP only, etc. You can find magazines and fliers listing these events at cosplay related stores. Its pretty much imperative that you know how to read Japanese if you want to be able to find these events.
There is also the JCF which hosts several cosplay events every month, usually at local amusement parks. These tend to be larger gatherings. I've been to 2 of their events and have enjoyed myself at both. They have a website and a newsletter you can get emailed to you listing the events for the upcoming month. It's all in Japanese, but if you run it through an online translator, you can usually figure out what's going on.
I've never had any problems while cosplaying in Japan. Most of the people I run across seemed amused to find a foreigner cosplaying and a few might even want to try speaking in English with you as well. Just be sure to follow any rules that a gathering has and you're all set. I also have more fun when I go with a friend rather than going alone. It feels a bit awkward going alone as everyone else always seems to come with a group. And it can be difficult meeting people if you have limited Japanese speaking ability.
Cons for 12’: World Cosplay Summit Parade - Summer Comiket - TGS - Winter Comiket
Finished for 12’: Irisviel von Einzbern, Fate Stay Zero - Asuna, Negima! - Queen of Hearts, Are you Alice?
In Progress: Queen!Yuuko, CLAMP no Kiseki - Queen , FF0- Illua, FF Advanced Tactics 2