Me, I just went on ebay and searched for pewter. This brings up lots of 
small hobby casting outfits but the radiator rebuilders offered it alot 
cheaper. 
 It's all, like, 99% or more plain tin and some small additives like 
antimony, copper, etc. Different refiners use different recipes but the 
additives are so small I can't tell any difference. You could indeed use 
staybright or modern soft "silver" solder for plumbing, but the 
expense..........
   The cast metal shrinks a bit when it cools so I build up the edges of 
the groove with a couple layers of tape first. Then, I lay a piece of 
greased twine (clothesline) in the groove before covering it with the 
masking tape. Don't stick the tape ends together but leave a bit of 
space.  This is so you can pull the string out after you've built up at 
least a couple/three  layers of tape on top of the string. Lately I've 
been building up cones of tape and drywall mud to form a small funnel 
for a pour spout. This gives you a bit more metal weight over the mold 
to push out a few more air bubbles (hate those bubbles). I also punch 
(or actually drill with an awl so's I don't crush the paper mold) a 
small air hole near the pour spout when the mud dries. Then tip it over 
a bit when pouring so the metal can flow around hitting the breather 
hole last. When I see metal peeking up through the hole I know it's made 
it. 
 You want the metal pretty hot. Too cold and it won't flow well enough. 
I heat over a one burner propane camp stove (elegance, eh? 8^) for at 
least several minutes after it melts.   If it scorches a toothpick right 
quick you're probably there. Graphite is supposed to help the metal flow 
better (George had better suggestions for pouring lube, but I couldn't 
find any of the stuff) so I'll pop a little squirt of it from a door 
lock lubricator tube just before I pour. I'm not sure it helps, but it 
doesn't hurt.   Last thing you do before tipping the ladle is a quick 
skim of surface crud with a stick.
   I still haven't been able to get it to flow into fine detail 
crevices. Rings or anything large is pretty easy. But get it down small 
and there's always some trouble. Last thing I heard was to pre heat the 
handle (it's wood, so I figured maybe a couple hundred degrees tops) and 
I'm going to try that soon. I'll let you know.
 yours, Scott
-- 
Scott Grandstaff, Box 409, Happy Camp, CA 96039 Tools: 
http://users.snowcrest.net/kitty/scott/scotts/tools/tools.html 
PageWorks: http://www.snowcrest.net/kitty/hpages/