OodlesOfPartsPlus.com and Swivel-Chair-Parts.com offer extensive directories of replacement parts for old office chairs. In addition, eBay.com sells replacement parts for old office chairs. Commonly sold parts include arm pads, base replacement, gas lift replacement, control arms and swivels.
OodlesOfPartsPlus.com features specific sections for Herman Miller, Guardsman and SteelCase chairs. Swivel-Chair-Parts.com features a simple diagram of the main parts used in an office chair with a directory of replacements for each part sorted by size and material.
Another route is to contact the original manufacturer directly. TriStateOfficeFurniture.com features a list of the top 100 office chair manufacturers in the United States. In addition, eBay features hundreds of auction listings for specific chair parts, which may include unique parts.