When I said create your own, this is what I meant
You do not have to go for those old notes that are now selling so expensive, if you can find them. You can particularly create your own list if you are a keen collector, plus a bit of spare cash. Below are few modern notes that I managed to get at a fair price and how they are selling at a ridiculous price: -
China - 100 yuan 2000 polymer. Back in early 2000, I exchanged a few of these notes with a Shanghai collector at face value against my AUD.
China - 100 yuan 2000 polymer 2-in-one uncut sheet. I only paid US$55 for this note. The last time I heard, the asking price was around $2,000
Poland - 20 zlotych 2015 P188 commemorative (30k sets issued). I paid a reasonable price (A$35) when it was first issued. Recently, a Polish collector asked if I was willing to exchange with him. This note is selling between $4-$500 a piece now
Poland - 20 zlotych 2016 P188A commemorative (35k sets issued). Similar asking price ($I paid A$45). Now the selling price is few hundred dollars.
Czech Republic - 100 koruna P28 overprint 200,000 pieces issued. A collector sold this note to me at face value back in May 2019, when I bought the P29 commemorative note from him. Recently, I saw someone selling on eBay for over $100+.
Malaysia 600 ringgit - This note was sold via public application. Only 6000 sets issued at 1,700 ringgit each (world largest banknote in size). I am not sure if this note was fully subscribed, but I believe many did not turn up and collect them, so it went for a second round, and they are all sold now. Just look at the ridiculous asking price now. A collector friend in Hong Kong sold one last year via auction, for US$6,000. I have 3 pieces and only paid RM1700 each.
Of course, I have few misses too; -
China - 10 yuan 2008 Olympic games commemorative (6 million pieces printed). On the same day, it was listed on eBay for US$99. Would you pay $99 for a note worth $2? I later managed to get one for around $200+ at a local shop here.
Ukraine - 50 hriven 2011 commemorative (only 1000 sets released. I also saw this item listed for $99 on ebay. I was not aware of the quantity issues then, so of course I did not buy. A collector in Ukraine not long ago offered me one but wanted 3 sets of Australia's current series of $185 in value in UNC condition. So the value of my notes would be just $555 against his. I believe, to him, his note is worth that much (catalogue value), but my AUD in UNC condition is only worth at face value.
See, I have few misses too, but this is how I created my special collection. Oh yes, I bought the Zimbabwe 100 trillion x 4 set with one ZA prefix, including a radar note for the 20 trillion note. I paid peanuts for them back in January 2010, now, it's worth $100+ each.