Fallen embers are indeed rare at higher level, where as on a new character, you get showered in them. That’s honestly the quickest way to get more - make a new character on a new realm.
Otherwise you have a few vendors that can have them which reset daily
- General Goods Vendor (lv 3 is best)
- Finley (only available 8-midnight)
- Bounties
- Challenges
- Crucible Vendor
But fallen embers are not ideal for creating a build anyway. See A Guide To Embers and Exalting So You Don't Waste Them - #2 by fishypixels
You have no control over what gets rolled or how close it is to max. Use essence embers instead for making builds.