Don't buy any cards that don't work with Mojave, like HD 5780/6870/6890 or NVIDIA cards earlier than Kepler. Apple killed support for these obsolete cards.
Buy one of these if you need boot screens:
- SAPPHIRE Radeon HD 7950 Mac Edition
- NVIDIA Quadro K5000 for Mac
- NVIDIA GeForce GTX 680 Mac Edition
You can buy the PC versions and flash, or pay someone to do the flashing part, or buy already flashed on eBay/MacVidCards. Read the threads about these cards and buy the one that fits your workflow best.