Not quite as cheap per u but you can get 10 packs of the APC 1u panels for about $50 on Amazon and probably cheaper on eBay. We buy these things by the case (of 400) at work, they are okay so long as you aren't constantly pulling them out.
If you want amazing blanking panels find the IBM panels with the rubber gaskets and metal spring clips on eBay, we have a number of those floating around that have to be over 20 and they still clip in cleanly every time.
APC rack blanking panel kit - 1 U - 10 PACK (AR8136BLK) https://www.amazon.com/dp/B0007RBWSU/
IBM blanking panel example https://www.ebay.com/itm/IBM-3-Space-3U-Server-IT-Rack-Mount-Server-Panel-Blank-Filler-Spacer-375739/192710670805