Although I've used regular infant scales in the past, including the
standard $14 "home nursery" spring scale (it gives relative weight and
is true to itself), I prefer the hand-held hanging scale (the Salter
spring scale) that can travel easily and has a hammock-type sling that
hangs from it for baby to lie in while being weighed . These are readily
available for retail merchants, but your best bet is through sporting
goods stores - ask for a "fish and game hanging scale". They come in all
sizes to weigh from small amounts (up to 6 pounds or 2 tons!!).

A quick 'Net search using (including the quoatation marks) "weighing
scales" and "hanging scales" yielded lots of links. Here's a few  -
(you'll probably have to make your own sling to suspend from the scale):

An American scale company:
http://www.chatillon.com/products/scales/

A tutorial  (a very good one, too!!) on human anthropometry with
information on scales including one from UNICEF:
http://www.odc.com/anthro/tutorial/tunit14.html
UNICEF hanging scale - (UNICEF catalogue no. 01 455 50) This is a Salter
type spring
     scale with a capacity of 25 kg and 100 g gradations.

A variety of "pocket-sized" scales:
http://www.yamatocorp.com/TIS.htm

Otherwise, contact one of the two best-known mail-order midwifery
suppliers (neither has a web page) at:

Moonflower Birthing Supply
(303)665-2120
PO Box 128 Louisville, CO 80027

Cascade Birthing Catalog
(503)378-7545
PO Box 12203 Salem, OR 97309

Etc. Well, I let you explore more if you like. (I used
http://altavista.digital.com/ )