Ebay is a starting point but it is a minefield. The best way to find them is people you know. In the 50s 60s and 70s everyone had mechanical watches (well almost the 70s). Your grandfather, uncle, or other distant relative may well have one in a drawer. Ask friends' parents and family friends. My Constellation Pie-Pan was from a casual acquaintance that happened to collect old Omegas (he never wore it, just kept them). I bought the Pie-Pan for about $850 or so, paid $150 to get some minor work done and regulation, and paid $120 or so for a brand new bracelet. The result is a very cool vintage Omega.

Very recently, I was looking on facebook and noticed a friend's dad in a picture, wearing a watch I thought looked like a vintage Omega automatic diver. I contacted him and we talked it over, turned out my first instinct was correct, the watch was an Omega Seamaster 120M "Baby PloProf" automatic diver. This mate's dad had gotten it in the early 90s from his father and had worn it ever since, probably not realizing what he had on his hands. I let him know about it and what to do to keep it in good condition as well as giving him a rough ballpark value. I didn't offer to buy the watch as I normally would as it no doubt meant something to the family and should stay with them but I now know that a valuable Omega has been saved, and will be cared for, and that really is all that matters.

They're around, thousands of them, in drawers, on the wrists of people that just consider them to be old watches, and owned by people you don't think would ever own a high end swiss watch. The fun part is finding them

 

As a 13 year old, your starting point is to learn everything you can.

Two best resources on Omega:

1: http://www.chronomaddox.com/

This is the best resource on chronographs and sports watches, read it all, and get to know what they look like. Look models up on ebay and watch sites to see what they are worth

2: http://omega-constellation-collectors.blogspot.com/

This is Desmond's site about vintage Constellations. These are watches with more class than anything this side of a Patek. Get to know them, read all there is, and absorb as much of Desmond's knowledge as you can.

3: Be patient. If you fall in love with something you may want to go for it, and there's nothing wrong with that, but the best deals are watches that fall into your lap. Keep saving cash, and have it ready to buy if something great comes along.

 

Make sure you price it out before you go spending much money, keen to see some pics though, the SeaStars go back as far as the 70s if I recall correctly.

 

There is a chance it may be a ladies watch or a "boys size" as they had many years ago. I'd be keen to see the pics, where did you dig it up?

 

Looks kind of cool actually. The bracelet is likely an aftermaket job from the 70s or 80s (I don't know much about Tissot, but it doesn't quite fit the case design). Looks bigger than 27mm, or at least wears bigger. Nice style actually.