The best way imo is definitely to buy the diamond and have the ring made to order - that way you can get a unique ring for about 60% of the normal price.
Main problem is that you REALLY need to do your homework on what diamond grades are the best for the ring, where/what type of flaws you can accept etc. For example if you buy yellow gold then you're simply wasting money buying a very white diamond grade as the reflection from the gold will colour the diamond anyway.
TLDR version: Get the best cut you can afford, with minor flaws not visible to the maked eye, in a colour that suits the setting.
Please remember you're buying the ring for a present, not an investment, it doesn't have to be perfect, just look perfect.
I got my diamond from James Allen in the USA, popped the question, then we both went to a good local jeweller with some designs she liked and created the ring from there. Total cost was £3500, insurance valuation is £5750 plus the setting is unique and exactly what my missus wanted.
Trust me, this will impress people much more than just having a huge low quality diamond in an average setting.