I have a lens (the Sigma 28mm F1.8 EX DG Aspherical Macro) but the lens hood is lost. Where can I buy a new one? Can I use any lens hood that fits my lens?
The part number of the lens hood for the Sigma 28mm F1.8 EX DG Aspherical Macro is LH825-03 and you can find it here. There are probably numerous retailers for it and you'll have to find one you like yourself.
Since the lens hood is designed especially for your lens, you can't use any lens hood that physically fits the lens. See this question for more information on that.
If you are on a really tight budget you can download a pattern and print it to make a hood for your lens out of stiff paper or thin cardboard. Just be sure not to resize anything when printing.