3D printing is a very efficient way to get as many minis as you want, but the initial cost is rather high in both money and time. Printers can be obtained for as little as $200 but please read reviews, go on YouTube, and check buyer ratings. There is a pretty steep learning curve as well so anticipate spending several days (probably weeks) just getting used to how everything works. It's a deep rabbit hole, but those that persevere can enjoy some immense freedom in printing out some really cool items for D&D and life in general.
A skeleton ready to take on the party. An easy victory, but perhaps there are 8 more in the wings.
These are some general resources for getting solid prints. In no way is this exhaustive. Since every printer has it's own unique settings and quirks you will need to Google your model for more in-depth information.
Listed below are sites where you can download and print dungeon decorations, minis, and dungeon tiles for free or little money. Thingiverse tends to be the one stop shop for most things, but all are worth at least looking at.
Shapeways is another big database of assorted models. The prices listed are only if you are going to order them from the site. Create an account and download them for free if you have a printer.
"The company supports creators with design tools and services, access to advanced production technology starting with 3D printing, and services to build a business."
"Thingiverse is a website dedicated to the sharing of user-created digital design files. Providing primarily free, open source hardware designs licensed under the GNU General Public License or Creative Commons licenses, users choose the type of user license they wish to attach to the designs they share."
Like Pinshape and Shapwways, Thingiverse is a bottomless well of models. Thingiverse tends to have better logs on what print settings were used on any given model.
"DesktopHero is an open-source, open-library character maker for 3D printing"
This is a free to use character designer in which you can download .STL files for printing. It's not as easy to use as Hero Forge, but being free certainly does have its benefits.
Fat Dragon Games
This site is primarily for interlocking terrain, but there are miniatures in their store as well. If you subscribe to their newsletter they will send you a free model every month.
"Fat Dragon Games has been in business since 2005, delivering the highest quality ‘print-and-play’ gaming terrain for more than a decade."
You can create customized minis through their robust editor. Aside from ordering the mini, you can purchase the .stl file directly from them to print on your own printer.
"Originally funded by Kickstarter, HERO FORGE® is a platform which allows you to create detailed characters in full 3D, right in your web browser, with an easy-to-use character customizer."
Miguel Zavala took it upon himself to model virtually every model in D&D and upload the designs on Shapways. Shapeways can be a little misleading though. There are prices shown if you want to have Shapeways print it for you all you need is an account and you can download the files for free.
"I'm an amateur 3D artist who absolutely loves playing Dungeons & Dragons and makes his own 3D Printable miniature files to share with the world for free!"
"We strive to provide our users an easy-to-use platform that lets them explore, share and download high-quality 3D printable designs."
Like Cults3d, YouMagine isn't as well known as other resource sites, but there are still many models to look at.
"YouMagine is an online community of 3D printing enthusiasts who wish to work together to share, remix and make better 3D printed things."
Cults3d is a resource site on which designers upload their models and you can print them out. It isn't as big as Thingiverse or Shapeways, but it's worth looking at.
"Cults is a marketplace that connects designers and people who want to 3D print some objects."
Pinshape is another database worth checking out for an assortment of 3d models to print.