When I need a mainspring for a cylinder music box movement, I go to
a nearby clock repair shop and buy one from them.  The last one cost
somewhere between $2 and $5.  I put it in myself (without the aid of
a true spring compressor -- if you try this on your own, be careful,
wear long sleeves and eye protection).  They would have put it in for
me for $120 -- 2 hrs labor minimum -- which would have included a
cleaning and lube (which I do myself).  I installed the spring, cleaned
and lubed it in about an hour.

I realize that not everyone has a clock repair shop (not a jeweler)
nearby, so here's the info on the shop I use:

 The Clock Center
 13219 Santa Fe Trail Dr.
 Shawnee Mission, KS 66215
 tel.: 1-913-492-3339

If you've removed the spring they'll need to know the length and the
width.  Not sure what else they'd need since I just walk in, show them
the movement, they disappear into the back room and come back with what
I need.

Also, there is a fellow on eBay that sells "movements for parts" --
18-tooth Reuge, recent manufacture -- for $5.50 including shipping.
He apparently repairs musical clocks (cuckoo) or bought out a cuckoo
repairman.  The movements are minus the regulator and gears since
(I've been told) that's the part that wears out on cuckoo clocks.
Look for seller "btomw".

I've also bought a number of complete movements off eBay, various
manufacturers.  I either use them for replacements of entire movements
or for parts to repair an existing movement.  Check it out.  Search
for "music box" plus "movements OR parts".

Good Luck.
Randy Faerber