Thanks for the kind words and for an awesome question.
Here’s what I think: Khakis and boots can and often do look awesome together. There’s some pretty big caveats to this though.
First and foremost, no pleats. Especially if you are wearing boots that are a bit pointy at the toes or where the toes pull up a little. You’re going to look like Ali Baba. Occasionally I’ll see some pleated khakis with round-toed, crepe soled boots and it looks okay, but mostly because it looks like golf clothes which is the opposite of what you’re trying to say in western wear, generally.
I’ll admit that I’m a bit biased about this because my Granddad and his bunch of old timers dressed like this, but high-waisted, flat front khakis look great with boots and a button down or conservative pearl snap shirt. Look at this picture of Gregory Peck hanging out in that get up:
Looks pretty cool right? That’s because the khakis have the same silhouette as jeans. There’s no pleats or cuffs or anything to alter the silhouette. It’s all straight lines.
Another reason why this picture is a good example of how to wear khakis with boots is the way the legs are hemmed. You want jeans to “stack” over boots, but what looks cool with thick, stiff denim just looks sloppy and bad with thinner, more pliable chino cloth. Hemming your khakis a little higher to wear with boots also allows you to show off the tops of your boots since, in theory, this is slightly dressier and you’re wearing fancier boots. This same principal works with other slacks, especially wool slacks (good to remember if you’re planning on wearing a suit with cowboy boots.) Not all tailors know how to hem pants to “break” properly over cowboy boots since the heel is taller than it would be on a pair of dress shoes. (The “break” is the dent in the leg of your pants where the hem meets your foot.) I’ve found that showing up in a pair of dress shoes and asking for a very slight break will usually give you the desired effect without freaking out your tailor.
Fabric weight is another thing to keep in mind. With thinner fabrics like wools and chino (chino is the cotton cloth that khakis are made from) I stick with dressier boots. Dressier boots usually fit a little snugger and are made with smoother, thinner leathers like calfskin. This way the texture of the boots and slacks aren’t jarring and too far apart from each other and the fabric isn’t straining against the tops of your boots, screwing up your crease. With heavier khakis and jeans you can get away with more functional boots made of thicker, tougher leathers.
So, to answer the question via the long way: I think khakis and boots go well together, as do other slacks. Western wear is based on workwear, country wear (think English country, oiled cotton coats, tweeds and plaids) and sporting clothes (think 19th Century idea of sports-equestrian sports and hunting.) Khakis have long been sporting and work wear staples, and in this context, they make perfect sense as western wear. I suspect the reason my Granddad and his buddies all wore these pants is because they weren’t far off from the uniform pants they wore in World War II. And, as whiskeydent noted above, it was considered pretty standard dress on the King Ranch.
As far as where to find khakis that look good with boots, I’m at a bit of a loss. I’ve been looking for some myself. I find that Dockers are either too low waisted and baggy or too snug and narrow in the leg. Depending on how dressed up you need to look though, plain old Dickies worked really well for me when I needed something that looked good with boots, looked presentable, and could take a hell of a beating. I knew they’d work for me, because they worked for my Granddad. And that’s a pretty good thing to know.