Make Your Friends Funnier
Do you know anyone you would like to write with? Someone who shares your comedy sensibilities and likes the same comedy that you like? Maybe you have a friend or acquaintance you think is hilarious, someone who makes you laugh consistently. Try writing a sketch with him. If he has absolutely no interest in becoming a comedy writer or has never written before, it might take some coaxing. You might have to ease him into the idea.
The best way is to ask him his opinion of your writing. If he likes what you write and thinks it's funny, that's great. If he makes suggestions that you agree with, that's even better. Try using the process of going over your writing to brainstorm new ideas. Start using that person as a sounding board for your ideas. If you and your friend are like-minded but there's still enough of a difference to make it interesting, ask him to write a sketch with you. Start with an idea that you've brainstormed together. Go out to lunch or dinner and throw around ideas. (Make sure you bring your notebook!) If the process goes well, you may have found yourself a writing partner; if it doesn't, try the process with someone else.
When you've found a comedy troupe you think you'd like to join, be warned that they may not be all that accepting to the idea of bringing in a new member or writer. Troupes can be very cliquish. Being too pushy will generally work against you.
Another approach is to look for an existing sketch troupe in your area. Watch their show and get a sense of the type of comedy they perform. If you feel that their style syncs with the material you've already written, ask them if you could submit some sketches for their consideration.
If the sketch troupe offers classes, take one or two. It's a great way to meet other comedians and writers, but it's also a great environment to show off not only your writing, but your personality and ability to work with others as well.
Finding others to work with can be a very frustrating experience, especially if they're used to writing with other people and you're new to comedy and don't have a reputation. If you find that you're not having any luck, you might have to take matters into your own hands.