You can use Printful's mockup generator to create your own mockup image for any of your products!
Simply pick your product and colors, add your design, click Generate Files, and then pick the mockups you want for your store.
If you want other mockups, you can use Placeit. Choose your shirt, upload your design, and you can have access to their library of mockups. We have a pretty sweet deal for you – when creating a new Placeit account through this link, you'll receive a 15% discount on single purchases for life or a 15% discount on the first month of any of our subscriptions.