Try Googling Kane Automotive, I bought mine from them. I bought a single gas analiser (CO) only, because thats all you can really influance if you tune the carb. make sure you get a stainless probe as the tail pipe temperature is higher than a car as its short. Its OK at idle but when you set the power valve on the Impco carb it melts the standard heat resisting rubber probe. But depends what you want to do. Prentice has a sniffer for measuring general CO in the air, normally used where people are working and you want to check if the air polution is too high. It would be no good stuffing that up a tail pipe!
Can I ask why you want to measure more than just CO?