Making changes in production for my templates and seeking ideas on creating a testing environment before moving to production. Maybe there is an app I could have my company install.
There is a testing/sandbox environment available, but it costs extra $$$ - like most things do in Salesforce. I HAVE heard of people being able to negotiate this for free, but that was before it was on the official documentation.
If you don't want to pay for the testing environment, then my recommendation would be to do like Sanchit mentioned and create a DEV/UAT environment via a BU. This, unfortunately, will still cost you for each message you send, each API/Automation you use, and each subscriber you store will still go towards your total DE storage and so on - as it would in your Production environment.
If you are looking to just test display/cross-browser support on emails, I would check out litmus or Email on Acid as they can provide you with screen shots of each email in dozens of browsers and email clients.
Apparently not. Here is the Exact Target DE Idea that you can vote on