Consider attending different shows, where you can find different performers as per your requirements. If you like one, do not hesitate to ask him/her after the show to join your group.
Take improv classes
You can also take local improv classes. The best way to know if you work well with someone is to actually perform with them on and off stage, and by taking classes you can hunt some pretty raw talent.
Post fliers at comedy clubs and theatres about your quest to find players. Always ask permission before you start propagating.
Search for people online
Consider looking online for people that want to do improv. Post threads in forums which attract professional and amatuer performers.
Try to get at least ten people in your group
The more people there will be onstage, better will be the energy. Remember that not every performer is going to make every performance. It is possible to perform with a group of three or four, but it has to be a fun energetic group, in which people support each other.
Develop characters for your group
It is important that some of the players have strong characters they can use in different scenarios. However, make sure that no too characters resemble with each other.
Rent out performance space
Check with local theatres, clubs and coffee shops if they have space available for your group to perform. Avoid any space that is too small or has poor lighting.
Start gathering wardrobe
You should always have an arsenal of funny costumes ready for certain games. Buy funny clothing or costumes from Halloween shops and arts/craft stores. You may also shop online from different websites.
Add music to your show
The best improv shows have music in between sequences. For that, you can purchase CDs or legally download music/ sound effects from different websites available online.