I look on e-bay for parts. for upgrades go to the Dell site and put in your serial number. this will tell you what is in it so you can get drivers for it. also look at the dell site for the 4100 series, see what optional upgrades you can get for it, then go to ebay and get those parts. or just search on ebay under "computers and network" "Dell 4100", this will give you everything you need, from parts to graphics cards.
here are the search results. have fun and enjoy the (ancient) laptop!