There are a number of vendors with suitable analog to serial devices, although much more common are analog to RS-485 rather than RS-232, and it is becoming more common to see Ethernet and USB as well as RS-485.
B&B Electronics sells the Advantech 'Adam' line (although they are RS-485), but have their own model #232SDA12, which is RS-232, but 0-5 VDC input, and not natively 4-20 mA. http://bb-elec.com/product_family.asp?FamilyId=107&TrailType=Sub&Trail=40 However, it is an easy matter to drop the 4-20 mA output across a 250 ohm resistor to give you 1 to 5 volts DC.
What you need to looking for is analog input modules which will accept mA input type and range +/- 20mA (such as ADLink ND-6017). In addition, you also need a RS-232 to RS-422/485 converter (such as ADLink ND-6520).