I would start my search at E-gun parts.com(Numrich Gun Parts Corp.).
Gun Parts Corp www.gunpartscorp.com
Try e-gunparts.com
ableammo.com has them for $13
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http://www.sellmystuff.com.au is the best place to buy and sell items.
Factory magazine for Marlin 80 DL $18 + $7 shipping at ammoclip.com
They don't have clips. They have magazines. You can order them through Numrich.
http://www.e-gunparts.com http://www.brownells.com 2 good sites
Try Numrich Gun Parts Corp.
you may wish to look at larger gun stores in your area for a used model 1892,or you can find some on gunbroker.com,or guns america.
You can get them from Marlin....
There are tons of places that you can buy the screw extractor from. Retail stores like Sears and Ace Hardware definitely has them. Amazon.com also have them available online.