You can find relatively cheap ones online. Damn near kilt em is one I can vouch for as a good first kilt. Or stillwater kilts for something cheap and more traditional looking.
The number one thing to know when buying is that you need to actually measure yourself. Pants lie, kiltmakers generally don't.
If you want something a little nicer, there are plenty of businesses that make kilts to measure. My last few came from alpha kilts, but I think they quit. Alt.kilt does crazy custom stuff. Utilikilt is probably the most famous one, though I've never actually tried theirs.
There's whole communities online for this stuff. I used to get reviews and find new stores on xmarksthescot.
If you get a contemporary kilt, meaning pockets and modern style, you just need a belt. If you go traditional tartan you probably need a sporran or some other kind of bag or pouch since they don’t have pockets. Traditional kilts also typically use a kilt pin to add weight to the front apron to keep it from blowing around too easily.
Tall socks are good formal wear or cold weather. I am a very casual guy and an arctic creature, so I usually just wear closed toe sandals. But if you live in a cold place and need to stay warm, a full size "9 yard" kilt will do the trick, it creates a pocket of warm air, and the legs being together gives it a mitten effect.
You can go commando (aka going regimental, because in highland regiments underwear is considered out of uniform), but I don't recommend it. For one thing, you don't know fear until you've been caught in a gust of 50 mph wind while walking past a school playground on a day that you decided to save time by forgoing underwear, and now you are desperately holding down all sides of a lightweight kilt and trying not to end up on sex offender registry. But on a much more mundane level, it's also just a lot easier to keep your kilt clean when it isn't directly touching your junk or your crack. I wear briefs but I go up a couple sizes, so that the elastic still holds them up but they hang off me when I'm standing, which feels like I'm wearing nothing at all.
Of course, if anyone asks what's worn under the kilt, you have lots of options:
Nothing's worn, it's all in excellent condition.
If you want to find out it will cost you dinner and a movie first.
I joked before, but your excellent answer made me seriously consider buying a kilt. I need to find out how the supply situation is here in Germany. Or I'll ask the seamstress that made my ex wife's wedding dress, she's working for the theater in my city as well. Maybe she'd like a challenge.
Remember, go up 2-4 inches from your pants size when buying a kilt. So, if you wear a size 32 pants, get a size 36 kilt. Their waist measurements are actually your waist measurement.
Also, the patterns have specific meanings. No one in the US is going to care, but if you go to Scotland someone may address you by the last name the pattern represents (they're like family crests). There are generic patterns out there, though.
I really want to wear kilts, but decent ones are really expensive and I don't want to spend that money on something I might not like (or more specifically I might not like bad attention from, because I do sadly care about that).