catalytic converter issues

[towncarblue]

my car is running fine without any issues, but i have the p0420 p0430 codes
i need to replace both of my catalytic converter. from rockauto the direct fit ones are about $400 after shipping [not including replacement costs ].
i can get two magnaflow UNIVERSAL catalytic converters for about $170 after shipping from a web store [not including replacement costs ] .

i was also looking at universal catalytic converters but i do not know their actual price or where to buy them . would universal catalytic converter work as well as the direct fit ? i need to get a inspection by 5/15/12 and my vehicle is throwing out p0420 p0430 codes. i was hoping that the 02 sensors gave out but that is not the case [ i replaced them already ] . the service engine light only comes on when i hit the highway , not during constant city traffic [ i keep constantly resetting the service engine light ]. I TRIED A PRODUCT CALLED " Cataclean " with no luck
those bolts look rusted out so i will not be replacing it myself, either way i can get it done for a good price.

i do not know which i am better off getting ? a direct fit or a universal? is there a difference?
where do i buy them online for a descent price?