Printable Details

Try sites like thingiverse, cults3d, etc...there are a lot of searchable sites where you can download 3D assetsm either as STL's or files that you can convert into STL's. Sometimes you may need to pay for a decent 3d file, but a lot can be had for free. I highly recommend fiddling with software like Tinkercad to build simple parts yourself. That really unlocks the power of the printer. 

There are a ton of free designs at sites like sketchfab. You can find an entire kitted out vehicle and delete everything but the kit! Even if an object isn't exactly what you want, if it is close, you can modify it to be more in-line with what you're after and doing so is easier than starting from scratch. Also this mod work will ease you into 3d design, it's how I learned - Also if you hit a wall YouTube is an excellent learning resource.