It can often be easier to find a critique group online than in real life. Some of the most famous include Critters and The Online Writing Workshop for Science Fiction and Fantasy. You can also search through yahoo groups or google groups for writing groups devoted to your specialization. In real life, try looking at message boards in libraries, taking classes at community colleges or universities, or putting out flyers yourself to meet other writers.
There are also organizations you can join, like the Authors Guild, Mystery Writers of America, Romance Writers of America, Horror Writers of America, and SCWBI (the Society for Children’s Book Writers and Illustrators.) For many of these organizations you need at least one professional sale. Check their requirements before applying.