Choose a theme for your retrospective. This either can be a collection of films by a single, prolific director, or a group of films that share a similar motif. Try to pick films that people would not normally be able to see on a big screen.
Contact local theater venues to see about renting a space for a night or day. Try to find theaters that are supportive of independent cinema; these theaters will be more open to retrospective programming. If there is a film festival in town, see if organizers are looking for a good idea to add to their list of films.
Touch base with a film distributor, such as Swank. Film distributors will have 35mm prints of many movies, although this will be for a price. Distributors also will sell you rights to show a film if you are using other formats, such as DVD.
Check the local library to see if they have any original film prints. Many public libraries house 16mm film prints, which can provide a very authentic experience for your audience.
Find funding by appealing to local businesses who might want to sponsor the event, or local catering services that may want to serve food. Keep your costs small by selecting only a few films to show in expensive 35mm format, preferably toward the end of the program.