I doubt you can buy Microsoft Windows and Office retail keys at a discount price. The only way to buy legitimate retail license is directly from Microsoft. There may be other official ways to buy Windows and Office but Microsoft hasn't provided a easy way to find them.
Note Microsoft provides special Retail volume keys for use by computer developers having a subscription. They are not to be resold. However, may of these keys probably end up being resold online. Also, these special Retail keys are like volume MAK keys because one key can be used to activate Windows or Office on multiple computers. For example, one key might be able to be used up to a 100 times before it is blocked by the activation servers.
You can buy cheap keys from a number of online sellers. They may be one of these special retail keys, an OEM key, or volume key. It is up to each person whether they want to take a chance on a cheap key or pay full price.
Note if one of these cheap keys does not work then many online sellers will replace them without question. Then you probably need to activate the product maybe within 30 days or the seller may not honor that offer.
BTW, over the last maybe 10 years I have purchased a number of cheap keys. A few keys failed to activate again several years after purchase. Since the keys were so cheap I just bought another one.
Also, I have had official retail keys fail activation several years after purchase. This includes Windows, Office, and software sold by companies like Adobe.
For example, last year after reinstalling Windows 11 my copy of Adobe Photoshop Elements 11 would no longer activate. Adobe support would not help me because that version was no longer supported. I had to buy a new copy of Adobe Photoshop Elements 2024. Note starting with Adobe Photoshop Elements 2025 the activation only lasts 3 years. After that you have to maybe buy a new license assuming they are available. They like a lot of companies really want you to buy monthly/yearly subscriptions. Microsoft really wants you to buy a Microsoft 365 subscription instead of Office 2021 or 2024. If they could they would make you buy a Windows subscription also.
You can buy the following Retail perpetual keys directly from the Microsoft store.
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Windows 11 Home (Download or USB) - $139.00
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Windows 11 Pro (Download or USB) - $199.99
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Office Professional 2021 - $439.99
* One-time purchase for 1 PC
* Classic 2021 versions of Word, Excel, PowerPoint, and Outlook, plus Publisher and Access
* Microsoft support included for first 60 days at no extra cost
* Compatible with Windows 11 and Windows 10*
* Works with Microsoft Teams
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Office Home 2024 (formerly Home & Student) - $179.99
* One-time purchase for 1 PC or Mac
* Classic 2024 desktop versions of Word, Excel, PowerPoint, and OneNote
* Access to support resources
* Compatible with Windows 11, Windows 10, or macOS*
* For non-commercial use
* Works with Microsoft Teams
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Office Home & Business 2024 - $249.99
* One-time purchase for 1 PC or Mac
* Classic 2024 desktop versions of Word, Excel, PowerPoint, Outlook, and OneNote
* Access to support resources
* Compatible with Windows 11, Windows 10, or macOS*
* Works with Microsoft Teams
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