Hi John,
As @danj stated, the 7000 indicates the grade of aluminum used in the watch. It is also, however, an indicator of what version of the Apple watch you have. On the very first model released, variously called "the original", Series 0, or more formally 1st Generation, that number is printed on the back. Although the Series 1 and 2 models are also made of that same material, that number isn't printed on the back; instead it specifically says "Series 1" or "Series 2".
So the device type your question is filed under, ***[[Topic:Apple Watch|Apple Watch - Original|new_window=true]],*** is correct. If you go to that page and scroll down to the Parts section, you can buy a battery there. The only thing you have to know is the size - 38 or 42 mm, which you've already said is 38 mm, so that's everything you need to know. Oh, by the way, the 1st Generation and Series 1 take the same battery, just as an FYI.
[product|IF308-001|A1578 Battery for Apple Watch (38 mm, Original & Series 1)|new_window=true]
For an extra $10 USD you can get a Fix Kit that includes everything you need to do the job; tools, adhesives and even a force touch sensor just in case. They don't always get damaged when opening the watch, but it happens often enough that it's worth having one on hand so you don't have to wait as you order one in the event it gets messed up.
Hope that helps; let us know how it goes!