Door prizes are usually items contributed by some company or individual. Usually they take time and effort to arrange, and you would like them to be given out "fairly".
See the
GiftStock Topic for a list of currently available stuff.
Probably the fairest method of giving out door prizes is to give each member a ticket, dropping the corresponding ticket in some type of container, then drawing the tickets at the end of the night.
On the other hand, there have been instances where other criteria were taken for giving out "goodies", such as asking "Who has been using Linux for X number of years, and allowing those groups to go first. This is not always "fair", and it descriminates against the younger members of the group ("younger" both in age and usage of Linux). This is an example of how we should be careful in disseminating prizes.
Contacts for asking for "contributions" are usually the marketing departments of the various distributions. Remember that even the lowliest doorprize costs money to make and ship, so be honest in how many you need and will distribute. Also understand that going back to the same company every month will probably wear out your welcome. One or two prizes every month, with extras at the
end-of-year meetings would probably be the way to go.
Remember that stockpiling Linux releases or programs does not work very well, as distributions get updated, and are therefore not as desirable as when they were new.