I swear we had this thread the other week.
Anyway as above they were an extra effort, made in smaller numbers than there are consoles (to say nothing of them generally being lost over the years because child's toy/electronics/all the usual), not made since. Limited supply, greater demand than supply -- there is not really a reason not to have one (not like the Saturn expansions where having one might trouble certain games) and they are used to make all the best games play in general or play nicer (some did like the Turok sequels without it for certain things) --
https://www.thegamer.com/best-n64-games-use-expansion-pak/ .
To that end there might be those looking to play one game that did not need the expansion and call that the entire use of their N64. However most will probably be looking to play all the games they can, and as mentioned many of the best the N64 had needed one, so they will be looking for one (in this old games in general I generally note you are facing people that had one back when and want it again, had a family one shared with siblings, played round a friend's/cousin's/whatever and want it, saw it in a magazine back when, resellers/investors, played it on an emulator and the general retro enthusiasts. No reason it would be any different here).
To that end time to work some overtime, learn electronics (if I can trick a raspberry pi into acting as a CPU and then some for an amiga this seems within reason, never mind just doing FPGA fun), hope you get lucky in a charity shop/car boot sale/facebook marketplace/... (good luck there -- I have not seen N64 stuff be vaguely common/get it out of my attic availability and prices for well over 15 years at this point but never say never I suppose as you might get some clueless grandma* price the one her grandkids left there like it was a decades old cooking gadget), emulate or see if the games you want are available elsewhere -- Rare replay on the xbone covers a lot of stuff here and individual Rare games have ports elsewhere, indeed I will play the original Perfect Dark these days as the XBLA version is so much better and the Tony Hawk games of the link above have things like
http://www.thugpro.com/ .
*I go to a lot of places where said grandmas are selling things, even they check ebay these days.