Start by reaching out to your entire staff and asking the simple question of whether or not they identify as part of geek culture and what Fandoms they are into. Staff is your easiest resource when it comes to ideas for panels and activities at your event, and you may be surprised at who comes forward.
Yes, your teen librarian is probably going to know a thing or two about anime or comics, but maybe the very buttoned up reference librarian is secretly a fierce and rabid Tolkien fan who might be able to host a killer pub quiz on all things LOTR (Lord of the Rings, for the uninitiated). Maybe the quiet guy from Periodicals has been collecting StarWars figurines since they were 5 and has a completely KILLER collection of items that would make an amazing display piece (under lock and key of course!). Maybe that fashionable new girl in circulation is an absolutely amazing seamstress who could do a teaching panel about how to read patterns, or basic sewing tips for beginning cosplayers. (BTW, I work with all 3 of the aforementioned people, and would never have known they had these skills had I not mined their brains for purposes of this program!)
Then, once you know the secret talents and strengths of the geekiest of your staffers, expand your circle of geekery to your volunteers. These are people who not only are likely to have cool hidden talents and fandom preferences, but also people who ALREADY love to help the library and might have some free time to dedicate to your endeavor.
Once you have secured help from within, it's time to head out into your town to find local businesses that might be interested in helping you with your event. Local comic book shops, gaming lounges, or anime stores are obvious places to go to, but think outside the box here too. Would a small local cafe want to sell themed treats to your guests? Would the local craft store be able to do a sewing teach or a jewelry making teach geared at a specific fandom? Would the party supply place down the street be willing to lend or donate masks or other items for a cosplay photo booth? Do you know a local photographer who might volunteer their time to take pictures or do so for a discounted fee? We have been lucky to partner up with all the aforementioned types of businesses in the past, and now, in our 6th year they are coming to us to ask if they can be vendors at our event.
Last but not least, hit social media to seek out local fan groups who are looking to recruit new members at an even such as the one you are planning. Do you have a chapter of the Madalorian Mercs, the Society for Creative Anachronism, Ghost Busters, or other historical reenactment groups? Do you have a group of local fiddlers who might enjoy dressing up as wizards, or jedis or whatever and playing a set for guests? Where are the local theater groups, the local live action role players, the local Dungeons and Dragons masters, the writer's societies, the artists guilds? No matter how small your town is there is always a group or two of aficionados of SOMETHING that would be interested in reaching a larger audience. It never hurts to ask them to take part.