It's not that hard to build one.
Here's the bare minimum that you need:
- CPU with a cooler
- Motherboard
- RAM
- Video card
- Hard Drive (solid state like m.2 or similar)
- Power supply
- Case
- Operating system on a USB drive or similar (Windows, Linux, whatever)
The other optional thing you might want is a modern built-in card reader to download your photos from the camera, but you can always use a USB cable from the camera as well. You'd have to make sure the case you buy has the slot for the card reader whether it be 3.5" or 5.25". You might need a DVD or Blu-ray drive, but that's fairly optional these days. The grand majority of people can get by without one. It'll take you +-1.5 hours or so to build it if you're not too familiar with building a PC.
If not, use Micro Center like already stated.