2001 Pre Runner Cat Converters

I bought a 2001 Tacoma Pre Runner a few months ago, 3.4 V6, 2WD, 4Door, 208,000 mi. Check engine light came on and local mechanic checked it out, said the code was for the cat. converters. Replaced them with aftermarket converters and light came on again. Took to Toyota dealer for diagnostic and said problems was with cat. converters, again (code P0420). They said most aftermarket cat. converters do not work well on Toyotas. They want $2700 for two new Toyota brand ones. Have any of you had success with any brand of aftermarket converters for this. Fortunately I live in a county that does not require inspection but I'd still like to take care of it, but not for $2700! Any input would be appreciated.