On line auctions, want ads, for sale ads, estate sales, garage sales, pawn shops, gun shops, gun stores.
Those that have been designed to do so. Winchester, S&W and High Standard have all made them.
Google the ammuntion and you will get over 10 pages of possibilites.
.22 Winchester auto cartridge is very different than the common .22 L.R.. The .22 Win. Auto cartridge has a larger rim and case diameter & won't interchange. The Aquila cartridge company (Mexico) made a special run of this oddball ammo & it is available in the U.S. from specialty shops for around $12-$15 a box of 50. Email me for cartridge measurements. I have an extra box of this ammo if interested.
That's the Winchester Model 490. A very high quality .22 semi-auto with the weight and feel of a Model 100. Built during 1976-1980. Pricey and hard to find.
22 Long Rifle
Anything labeled 22 magnum, such as a Henry Golden Boy.
Your winchester model 1903 shoots a different ammo than regular .22cal rimfire made today.It is chambered for the .22 Winchester auto rimfire cartridge.This ammo can still be found I believe at Old western scrounger. Try www.old western scrounger.com.I believe they are 12.95 a box for 50 rounds,maybe a bit more.
Several different manufacturers make it (e.g. Remington, Hornady, Winchester), but there is no such thing as "inexpensive" factory loaded 22 Hornet ammo.
CCI .22 Long Rifle- or any standard .22 Long Rifle ammo (there has been a major shortage of .22 ammo for a year now)
The sears model 6c-22 was made Winchester/ same as Winchester model#490
You can find everything you need at Harbor Freight.
Only if it is chambered for it. If you are not sure, take it to a gunsmith.