You may want to check with your local jewelers as many sell pre-owned watches. Similarly, you can Google the web for "pre-owned Omega Speedmaster" Tourneau is a very reliable seller of pre-owned watches and as a national chain, they may well have something in one store that can be obtained by the one close to you.
http://www.tourneau.com/mobileCatego...ned/rvids/5103
Here are some helpful resources:
http://www.ablogtowatch.com/collecti...ega-watches/3/ &
http://www.ablogtowatch.com/collecti...ega-watches/4/
Here are some other sellers of used watches, but I have no sense of them or whether to buy from them. I think it safe to say that if an outfit has a brick and mortar location, they are likely a good place to buy from.
http://www.chrono24.com/en/omega/pre...mod71-used.htm
http://www.tic-tock.com/wingates/wat...edmaster/Mens/
Lastly, I will say that I am always very skeptical about buying pre-owned watches over the www, particularly pricey ones. There are just far too many easy ways to get taken and that would tick me off a lot if it happened and it would be very hard to obtain recourse if it did. Just as a simple example, the seller could easily have an authentic watch and provide you tons of pics of it, but actually send you a fake.
Several things that make me concerned:
1 - Seller doesn't accept credit cards.
2 - Seller doesn't accept checks.
3 - Seller doesn't use PayPal or some other payment mode allowing the buyer some recourse.
4 - Seller only wants wire transfers
5 - Seller asks you to mark the payment as a gift, for any reason. You lose your recourse when you do that.
6 - Seller is outside your home country. Could cause customs issues.
Good luck and all the best.