Availability of Icons
Commercially available icons only try to cover the most general needs as the other needs could be really specialized. For example, if you are creating an animal habitat map of coyote population just outside your town area, the need for icons are different and not likely to be in high demand. Such special classes of icons usually are not available widely. Many a times you may need to roll your own symbols/icons for such projects.
What is available is a pack of icons arranged in categories. Typically they sell in the $100 to $250 range. They are royalty free, usually, for distribution. If you needed only 1 or 2 then they will cost you few dollars a piece, $5 being a common number quoted at the top sites. Icons-Land has designed a package that is quite popular. It is being offered by several download sites and are backed by reviews. GIS/GPS/Map Icons usually need to be distinct and kind of jump out of the page so that users can find the type of object of his interest. The icons have to represent different classes of objects, of course. Users would then be able to orient themselves and search and find the points of interest. The icons offered by Icons-Land are available in six groups. Categories they have named as pinpoints, places, containers, GIS, devices and a Misc group. Pinpoints contain symbols used for marking specific places such as objects of various sizes and colors, such as bulbs, flags, needles. Places group contains icons that indicate places such as Hotel, Bank, Hospital, Airport, Gas station etc. Containers group indicate various containers which can be used in GPS tracking applications for cargo and assets tracking. GIS contains icons relates to items used typically in GIS applications such as region selection, coordinate systems, layers, etc. Devices group contains devices that we use, cell phones, PDAs, and so on. Misc would be a group containing all the rest of icons and could be road signs, traffic lights, and so on.
These groupings and the nature of diverse icons availability is similar in other available products. The sizes the icons are available seems to be fairly standard. The sizes typically are 16x16, 24x24, 32x32, 48x48, 64x64, 128x128, 256x256 pixels. Another thing common is the format of these images. The image format is PNG in most cases. Some sites will give you ICO format icons also. Transparency settings are possible.