If they are converted coupes then it may be real hard to get true numbers.
The early models were custom made from coupes but I thought somewhere around 1985 they were actually made that way at the Toyota factory in Japan.
They were certainly offered to the Japanese public with K in the build code instead of the usual C or S for coupe.
The closest I know of for production numbers is at
http://jnc.farpost.com/data/framno/longindex.html
But that will only help you if the convertible has a different model number than the coupe.
You can try asking at a Toyota dealer parts desk.
They have CD's that tell them the body serial number for each model by month.
- Stepho