Car rental types are generally defined by SIPP codes, which is a standard defined by a consortium of car rental companies. These are 4 character codes that indicate the size, type, transmission/drive, and fuel type/air conditioning. For example, ICMD represents an intermediate (I) size 2/4 Door (C) car with manual transmission (M) and diesel with air conditioning (D). ICMQ would be the same without air conditioning. Incidentally, this is why you generally don't get to choose a specific model of car - you've actually booked any car that happens to match a particular SIPP code, hence the "VW Golf or similar". So the good news is that it's possible to tell a car rental company what type of car you want to a fair degree of precision.
The bad news is that websites generally won't expose these codes to you, especially the last character. Off the top of my head I'm not aware of any site that lets you explicitly search by SIPP code. Your best bet is likely to communicate directly with the local rental office. It's also possible that a travel agent would be able to book a car by SIPP code, if you want to go that route.