I am looking to buy a plant or cuttings from a young plant called
Sarandi Blanco (it's a tree or bush). The botanical name is
Phyllanthus sellowianus, from the botanical family of Euphorbiaceae. 
I understand its origins are in South America, sometimes found along
the riverside of Parana, Uruguay and La Plata rivers.  I would like
listings of companies that sell this specific item. Capsules or
tablets from this plant would be an acceptable alternative. This plant
is known to have positive effects on diabetes.  Thank you.

I don't know if this will help anyone in finding your answer but
Phyllanthus sellowianus is no longer a species in it's own right, it
is now thought of as a variety of phyllanthus niruri. I will try and
find a paper for you describing the change in classification of the
plant.

This site includes a list of all the healing effects of:
Family: Euphorbiaceae
Genus: Phyllanthus
Species: niruri, amarus

Including the variety sellowianus, it is more commonly known as chanca piedra.

http://www.rain-tree.com/chanca.htm
http://www.singleherbs.org/products/bhuimala.htm

It seems that it is just as common in india and used regularly there
as a medicine, as much or more than it is in south-america where it is
often known as the stone breaker for it's ease in breaking stones in
the body.

Available on these sites in capsule / tablet and liquid form

http://www.americasnutrition.com/chancapiedra.html
http://www.shop-com.co.uk/amos/cc/main/aprod/act/p13242422/sourceid/13
http://www.iherb.com/phyllanthus1.html
http://www.raintreenutrition.net/products/chanca-piedra-extract.htm
http://www.weight-care.com/herb_chancapiedra.htm
http://www.musclesurf.com/phylantus.html