Tell them your situation, i've only trained at one gym so I can tell you my experience. In my case i went in after work one day talked to the coach, emailed him first to make sure he'd be there. I asked about classes told him i'm a full time student and I work so it would be hard for me to train regularly. He said they do classes for $10, so I went to a class, he showed me some basic movements and how to hit the speed bag. After class he told me that I could use the gym for $30 a month (it later dropped to $25, I laughed that usually fees go up, not down) and said they open at 6. So I started going at 6 regularly before work, when the gym wasn't busy and working what he showed me. Eventually he showed me how to hit the heavy properly, I worked that in addition the the first things I learned and he would tweak me up on occasion. Now I pay the $20 for an occasional private lesson every so often and he continues to tweak me up when he's not busy. I've train martial arts for yeas before taking up boxing but I only work boxing and proper technique when I'm there, I avoid trying out other stuff I know and keep a humble attitude and it's worked great for me. So find a place tell them what you want to do, see if they'll let you train on occasion or attend a single class for a small fee.
If your not set on kick boxing/boxing or if it doesn't work out look into Filipino martial arts, aka kali, arnis, escrima, its not a very popular art and many groups just like to have more people to train with. A lot of FMA happens in parks, garages, and subleted space in larger martial arts schools. I teach and am glad to have anyone that shows interest. I charge a little and would teach a student that showed some dedication for free, but it's rare to find people that motivated.