Memory Storage

Where can I get access to extra memory for my iMac?

Depends if it is a desktop, go to Scorptec computers is where i purchased 8 x 128GB 3200 speed DDR4 RAMs Corsair vengence LPX, if that helps.

Which model?

iMac (Retina 4K, 21.5-inch, 2019)

If you want absolute peace of mind knowing that it’s the exact right kind of memory then just go here:
https://eshop.macsales.com/shop/memory/imac-2019-21.5-inch

according to everymac
https://everymac.com/systems/apple/imac/specs/imac-core-i5-3.0-21-inch-aluminum-retina-4k-early-2019-specs.html

So if you are willing to shop around you need to look for PC4-21300 DDR4 SO-DIMM. That is basically smaller form factor memory typically used in laptops. The number defines what speed it is.