you may try xtr1xx (xtr101, xtr103, xtr104, xtr105,...) from burr-brown to convert voltage to current and using rcv420 also from burr-brown to convert current to voltage.
just becareful that some of the xtr model had obsolated, please go to their site and find it out. good luck.
check MAXIM design guide **broken link removed** there is very interesting idea for 4-20, or 0-20mA source, different from AD421 and XTRs. I think it could help you. there are a lot of solutions with this ideas, I use MAX535 and it is much chaper!!!