No. You do not have to purchase or buy additional memory from Apple.
In fact, I have made an entire list of memory upgrade stores other than Apple that you can safely and easily buy memory upgrades for your Apple Computer that offers a much better value than if you were to buy memory upgrades direct from the manufacturer.
To be sure, buying Apple memory upgrades from Apple directly offers benefits that others on my list can’t compete with, and that’s probably support issues. If you have Apple memory bought from Apple directly and installed (usually at time of your Apple computer purchase) all at the same time there is less of chance of trouble when and if the time comes that you do need support for your Apple computer. If you have Apple memory installed at the factory, Apple cannot use the excuse that your computer problems are due to bad ram from third party memory vendors. The problem with buying memory directly from Apple you will pay dearly for the pleasure when you compare ram prices from Apple against much cheaper memory upgrade options where you can save up to half price when you compare third party memory to the Apple Memory.
If you stick to a trusted memory store that’s been in business for over a decade and specializes in Apple ram upgrades you will still get the benefits a ram upgrade can bring but for much less money than had you bought the RAM from Apple directly. Most (actually, all) memory stores I track prices for and monitor offer lifetime warranty and have been in business for at very least, a decade and in some cases like OWC, over twenty years selling you memory upgrades for your Apple Mac computer regardless of whether you’re in the market to upgrade an older Mac or maximize the RAM on a new Apple Computer.
Buy RAM from a trusted memory store and you’ll save money on your next Apple RAM Upgrade.