This is interesting to me. We avoid group tours as we are not afraid to drive ourselves in other countries and we enjoy the adventure of exploring on our own. However I’m sure we miss things sometimes that we would learn if we hired a local guide. How do you find your local private guides?
Well, we have done it several ways. Sometimes we drive ourselves, but we limit that to countries that use the Roman alphabet. My Arabic, Tamil, Vietnamese, etc. skills aren't good enough to read street signs.
Our first resource for this is International Travel News. ITN publishes articles and letters from readers who report on guides and tour companies. I search back issues on line and have found these reports to be very reliable.
So .. where we're driving or on a river boat, we go to the internet and browse for local guides in their towns. There are web sites that list guides, but we look for guides' own web sites and emails. There's no point in paying a middle man, plus I think the more popular and higher quality guides do not need to list themselves in a "meat market" type context.
Most of the time we travel independently, though, we find tour companies in the destination country and work out an itinerary that suits us. I go in-country to eliminate the middleman fee of a US agent, plus a US agent eleven time zones away is not going to be much help if a problem arises.
After we've picked a country I go web surfing. With most tour companies the web site is clear that they only do groups and/or they only do fixed itineraries. Some, though, mention doing "private," "custom," or "independent tours. We then inquire to them. Most come back with fixed itineraries or communicate some kind of rigidity. A few come back sounding interesting, so I downselect to just two based on various factors including cost, then start working on itineraries. In maybe ten trips like this we have never had a "failure." The ITN articles are a big help in this success rate. In January we went to Vietnam and Myanmar, selecting an agent who had been recommended multiple times in ITN and the trip was great. Her reviews were so good that I didn't even look for competitors.
One of the things I always ask for is a couple of references from customers who have traveled with the guide or company within the last two months. (Everyone has a few good references in their pockets but those are typically not recent.) I then contact the references and attempt to have phone conversation with them.
Sorry so long. Hope this helps.