I often check at the foreign exchanges at the airports I go through for interesting notes, often they have the newest issue unc notes. It takes a bit of tact tho, I tell them upfront that I'm a collector and am looking for new issues of notes.
I also tell them that if they get a legit customer I'll stand aside and let them look after that person right away, then they know you aren't going to clog up the system while they have multiple customers waiting.
Don't bug them about the service fee or commission, quite often if you buy a few different notes from a few countries they will combine or charge only one fee, but if they don't, and they charge you full fee, its still cheaper than Ebay or the Dealers, plus the cashiers get a cut of the commission and if you hassle them about the fees, they are less likely going to be willing to help you out.
Have an ALPHABETICAL List of the most important notes you are looking for, they want to know exactly what you are looking for and if you start out with an "A" country, and then an "E" country, they will pull out the bundles and you can often get a quick peek at the other things they have. When they find what you are looking for, its ok to ask for the best one possible, I've often taken one that has been circulated when there are no other better just to get an example and then once I can get a better one I can use the lesser to show the back while the nicer one has the front displayed.
Always check the notes for stamps pin holes and writing, its ok to change a note if you see writing on it, just do it before you pay so that it doesn't cause them a headache to redo everything after and Pin holes are a fact of many notes from South Asia as they come from the banks stapled in bundles.
Always ask if they have anything Odd and Unusual, I've gotten old Scottish pounds (1950's and 60's) Odd ball notes like 30 Dinars from Tunisia, small denomination notes that are too small for their system to exchange, and on occasion replacement notes from other countries.
90% of the time the teller has no interest in collecting notes, but sometimes you find one that is interested in learning a bit, don't overwhelm them, just chat and share info and respect their time that they are spending with you.
Plan to spend $50 and tell the cashier that up front, then they know you are planning to spend a few dollars and will take time to look after you.
I don't restrict myself to only unc notes, I love XF and AU notes, a bit of wisdom from a note collector with one of the best world banknote and Canadian chartered banknote collections goes to the tune of an AU note is just an unc mishandled once, an XF mishandles a few times more and looks just as bright and colourful, but half the cost from a dealer. Its nice to find Uncs, but I like handling the notes and being able to show them to friends and other collectors, and I never have to worry when they are XF or AU circulation notes. Sometimes the circulated notes have a charm of been used in the places they have come from rather than just have been in a bank vault after coming from the printer. I often wonder what the roman Dinarii that I have in my collection has bought when it was circulating around as money back then, I don't have to wonder what the mint set or the Unc notes have bought or done in their lifetime, they've just been sitting there looking pretty.
And lastly, always respect a "no we have nothing of interest" and let it go at that, smile and wish them a good day and move on.
I've found tons of notes this way and ones that I'm happy to handle and look at enjoy, and have had lots of fun snooping in all corners to find them.
Good luck in your hunt for treasures!
