For only a few games i would indeed recommend to just buy a sports game for a couple euro's etc. Or lose empty case wich you could buy them in bulk for 20 - 30 euro's per 10 excluding shipping factory sealed even more people are pretty desperate these days, If your going online shipping will still be pricy even for a cheap sports game and locally you would need a source not to mention the luck that a nice case will be there 50 cents for a ps1 game is a rarity so usually it will be a couple of euro's at least.
It is however not the most cost efficient way by a long shot in my opinion. Since the game is basicly trash unsellable unless you have many in a big lot in wich case you still lose money. It's a quick fix nothing more nothing less. However the most cost efficient method is only recommended if you have patience and your main focus is not just spare cases not to mention if you are going to seriously collect for the system and thus have many games wich need replacement cases.
The definitive way in my opinion is by buying cheap sets. Keep the games that you want and sell the rest for a profit. Spare cases are just a bonus when done that way, that is the best way to replace cases. In the beginning you will have allot of cracked cases but as time goes by you'll catch up. Have replaced all my cases with pretty much perfect ones this way not to mention keeping some trash games aside with perfect boxes for potential future replacements since they would not ad much value to the lot of cheap games anyways.
If you have the cash and want it quick just buy some online or wait for some sealed bundles to come up.
Here a listing for 5 pieces
https://www.ebay.nl/itm/5-X-PS1-REPLACEMENT-SINGLE-PAL-GAME-CASES-Playstation-Game-A/253555870245?hash=item3b091b8a25:g:bpgAAOSwdGJazoIKPlastic static Boxes have been fully tested and are in working order they lost me with that one
They are not original btw but they have the same sizes, original cases do show up from time to time but they are pricy and since it's not always available people can be pretty desperate. it is usually only sold in a minum of 10 orso when they do show up.
