Where do you live? Moss that would be good for growing on Bonsai pots can be found just about all over. I live in a condo and I can just about always find little pieces under drain pipes in the parking lot. Just use a butterknife to lift... If not there, then I look in shady spots along the road or parking lot... Sometimes on pathways under trees between the rocks. You can collect several small pieces to cover the pot... Keep it outdoors and cover with a piece of opaque plastic and it should continue to grow to cover the pot nicely. Unless of course you have an indoor bonsai or one that can not tolerate your winters, then just keep it misted. I'm sure others would have other advice as well.
Hope that helps.

Thanks for the info.i have taken some moss from my backyard and it died in the pot.though it was in the middle of summer when i think it was already dead.now that it has started raining here i think ill try again.the bonsai i want it for is a ficus and golden rain tree.again thanks for the info

HI, Using moss in Bonsai can be really rewarding, but difficult. Moss requires extremely high humidity and does not tolerate on-going fertilization. However, that being said it is so plentiful that you can often just replace it for showing. To keep it alive it's important to mist it with rain water or distilled water, rainwater is best as it gets it's food from the air so to speak. Misting should be done two to three times a day to maintain optimum humidity, especially when subject top indoor conditions. Also if you use moss that is attaced to wood that works better as you can keep the wood under the soil surface as need be or incorporate it into your design. Rule of thumb, whatever the moss is already growing on is what it should remain growing on. good luck