To provide your own template files, you will need to host your files somewhere accessible by the Portainer Server instance. This could be somewhere like GitHub, a web server, or perhaps a container running nginx.
As an example, the Portainer templates repository includes a Dockerfile that lets you start it as a container to serve the JSON file. To set this up, first clone the Portainer templates repository, edit the templates file, then build and run the container:
git clone https://github.com/portainer/templates.git portainer-templates
cd portainer-templates
# Edit the file templates.json
docker build -t portainer-templates .
docker run -d -p "8080:80" portainer-templates
Access your template definitions at http://docker-host:8080/templates.json.
You can also mount the templates.json file inside the container, so you can edit the file and see live changes:
docker run -d -p "8080:80" -v "${PWD}/templates.json:/usr/share/nginx/html/templates.json" portainer-templates
For more information about the format of the app template, go here.