I made some years ago , dont think I have any left. It just had an image of a detector I got off the web and said something like "Item recovery , if youve lost something I may be able to find it. Metal detector training/instruction available upon request. ( this last sentence may open doors from those who are interested in learning and could open up their yards/property to you )
You can buy business card stock at many stores and there are computer programs available to print your own. If you did it this way you could easily change your design or information and print more up at any time. I think its worth saying that using too many words or description can turn people off and they do the rolling eyes thing dismissing it as silly since as discussed in atleast one other thread on here public opinion of metal detectorists is usually bad or that its a hobby for old people with too much time to waste. If kept as short and professional looking as possible it may give more of a professional "business" look to it and be taken more seriously.