We've used anything from plastic pipe to bronze thrust washers or bushings (cheap) to shaft collars. One time, a plastic cigar tube happened to be the perfect fit. At the end of a shaft, or for precise positioning, shaft collars work nicely. They are relatively inexpensive at MSC, and are available in aluminum or steel in one- or two-piece designs. I highly recommend having some around.
MSC shaft collars (starts on page 3817)
http://www1.mscdirect.com/CGI/NNPDFF...0&PMT4TP=*LTIP
various bushings and washers (starts on page 3772)
http://www1.mscdirect.com/CGI/NNPDFF...0&PMT4TP=*LTIP