Welcome to the Spencer Shooting society. You have a mid range serial number Spencer Carbine that was probably used during the civil war. If it has not been altered by Springfield Armory and still has 6 grooves in the barrel and no cut off switch near the trigger, then probably your carbine has a .52 caliber. It is best to slug the bore to find out exactly. It is also advisable to have a antique firearms expert check the gun out to make sure it will not explode on the first bullet fired out of a 150 year old gun.
That being said, (Caveat Emptor), I got mine checked out and it was still a .52 caliber with 6 fine grooves. I bought my parts of bullets from Buffalo Arms.
http://www.buffaloarms.com/ I made a couple posts here about my journey in making bullets and firing them. No company sells rimfire bullets for the Spencer anymore. You have to get a centerfire conversion kit from S&S in New York.
http://www.ssfirearms.com/proddetail.asp?prod=SP176 They also have bullets and blanks and spare parts.
There are several online shops, but those are two I have used successfully. You can also search for blanks here on this forum, as I have used them for a re-enactment with nactorman.