G.B.
You might call the court house and see if you can get in to see a judge to get married. It is usually in the smallest court room and all your guests could go. Otherwise maybe you could just call the Parks and Recreation department of your city and see if they will let you get married at one of the City parks. By bringing a few chairs you can set up a small intimate area that is perhaps in a pavillion, gazebo, or some sort of makeshift area in a small clearing or surrounded by gardens.
If you decide to try outside then you can get arches and other decorations at stores like Michaels or Hobby Lobby. You can also get those 10 foot square tents/canopies that fold up and can be carried to give shelter for that many people at Walmart for about $20. We bought one for BMX so that we can pop it up and have shade when we go to the track or travel to tracks that don't have shade.
If you go to Walmart go to Sporting Goods and find the chairs that fold up in a bag to be carried. The canopies should be in that area. My friend got hers there even though walmart.com said they were not sold in stores. They fold up and are carried around in a bag too and are lightweight. It's just an idea in case you end up being out doors.
Good luck on your marriage.