There are a handful of companies that ask for work references. You will find through forums folks with whom you build a mutual trust over time. These are the folks that you communicate with outside of the forum arena itself as well. Develop some of these friendships and use each other as references for those company sign ups. On only one occasion have I ever gotten a call about one of the ones I have allowed to use me as a reference. I could comfortably tell them that I knew she had been doing shops 'at least' the number of years I have known her as a shopper. I could tell them the kinds of shops she liked to do most and that from my experience with her she was quite reliable.
In many cases it is not advisable to notify your work supervisor that you are doing free-lance work as well. Only you can be a judge of that. Sometimes it can lead to salary discussions that let your regular employer know that you like working for them but your salary just does not cover your expenses so you are finding it necessary to 'burn the candle on both ends'. I would never tell a regular employer that you are doing this 'for fun'. But if you are going to tell them I would certainly do it BEFORE listing them as a reference so they know you aren't 'job jumping' but rather doing fill-in work on your own time.