You can get on of these at most shooting sports stores. You should try Cabela's first
It appears that you have a Ruger Mk I, manufactured in 1978.
Sturm Ruger still makes magazines for that pistol. They are the same ones used in their MK I variant and have a 9 round capacity. The subsequent variants, the MK II and MK III . They can be purchased on-line at the Sturm Ruger website, and are commonly available at gun shops.
Ruger made a MK III, not Remington.
Ruger has never been made outside of the USA.
Ruger has extensive sn data on their website. You will have to go there and look it up.
50-290 or so
Ruger's web site has sn data on it.
You will have to call Ruger. You can get the phone number from their web site.
For this particular pistol - Mk I, Mk II, and Mk III - it is highly recommended to let a professional handle this. Many reassembly mishaps have resulted from DIY-ing it with these pistols.Contact Ruger Customer Service at the website, get an owner's manual.
You can request an owner's manual for free on Ruger's web site.
If it's a .22 Ruger semi auto pistol, it's probably a Mk. I, Mk. II, or Mk. III. See related link 'Sturm, Ruger & Company'. On the left side of that website, there'll be a menu of their products, including their rimfire pistols. Click on 'Rimfire Pistols', and you'll see a drop down menu which will have their rimfire pistols, including the Charger and Mk. III. The Mk. I and Mk. II are simply older versions of what the current Mk. III is.
Of course it depends on condition, whether its a MK-I or Mk-II, etc. but the value shows around $300 average.