If something is hard to come by, it's rare or difficult to find. This means it's not ubiquitous, and you often have to look for it. This can be things or types of people or personalities. For example, you can say a clean public restroom is hard to come by when you are walking around town. Similarly, when you come across a mean worker at a store, you can comment that good customer service is hard to come by nowadays. You usually use this phrase when you notice that something is rare and that sometimes, you wish there were more of it.
A: Thanks for watching our kids last night.
B: No problem. They're so studious. I was surprised. Well disciplined kids are so hard to come by these days.
A: Thanks. We told them they were supposed to do their homework before bed.
B: So how was your friend's party?
A: It was great. We had a lot of fun and the food was fantastic. My friend had been a little worried because she'd had a string of bad caterers.
B: Really, my friend said the same thing. It sounds like good caterers are hard to come by these days.
A: It seems like it. I wonder why.
rare - not common
ubiquitous - seen everywhere; very common
come across - encounter; meet
mean - not nice; not kind
nowadays - these days
studious - someone who studies hard
caterer - someone who prepares and serves food for a party
Alright folks, keep this expression in mind the next couple of days, and when you find yourself looking for something that is rare--a good movie, a cheap plane ticket, a helpful person on the street--you can comment that it's hard to come by.