Where I can buy this converter in USA ?

HXSP-2108E-A:HXSP-2108E-A RS232 to TCP/IP Serial Server

http://www.hexin-tech.com.cn/HXSP-2108E-A_RS232_To_Ethernet_TCP_IP_Serial_Device_Server-pnb-207-en.html

I can tell you that these units work fine, have about 4 of the RS232 to TCP/IP units. Bought them last year whilst i was in Shenzhen for work. There are other alternatives such as the LAntronix XPort, these are resold by many manufacturers into there own convertors.
http://www.comfiletech.com/index.asp?PageAction=VIEWPROD&ProdID=24
http://www.alphamicro.net/components/product~line~4~id~223.asp

The device I mentioned is also manufactured by Shenzhen , I ever use XPort series , But now I would like cheaper .

Searching for "Terminal Servers" should yield some fruitful results. There are several manufacturers of them with prices all over the map. Other manufacturers are Moxa and Digi

If you have them in a single area and depending on your application, you might be able to pickup a single 16 port unit that provides a single tcp/ip session for control of 16 ports. You can find them used on ebay for less than 200.

smarthome.com

They sell them

RS485 is widely used in electronics system. PLC-RS485 transceiver module from LinkSprite (LinkSprite module) is transceiver module designed to send/receive RS485 data over the powerline network. PLC-RS485 is designed to transparently move serial data over the powerline network.

LinkSprite modules have the built-in packet-level repeater function. This feature can greatly extend the coverage of the powerline communication.

LinkSprite modules has both physical and logic addresses. In a network, both physical and logic addresses can be used to address different nodes in the network.

see linksprite dot com for details.