If you are after specific power source and nothing else, then treasure of the sages (usually).

Advnced refines - treasure of the sages.

Deluxe charm material pack (5x50 for 4k) in the shop usually duing charm rebates. If you want specific charm fragments- treasure of the sages and optional charm pack with discount.

Tactics - konoha outlet (225/box).

Medium refines- get them for free from monthly/group shop (takes about a year of getting them for free to pass that level)

medium threads- monthly, sa, space-time redeem section

purification breakthrough pills- top places during events, x500 refine rebate and 8 spins during myoboku wheel (really bad investment, dont do that)

advanced threads- sa, top8 spacetime during fight week, spacetime shop. Currently theres no good event to get them in larger ammount (low power income compared to cost)

cave keys- cave key gift pack in fruit event or in treasure of the sages with discount. Around <800cp is ok price.

myoboku gifts- treasure of the sages, konoha outlet (not sure about that one).

Mood scrolls- 10/1 in the treasure of the sages, optionally going wheels and getting some of them (see below)

Essences- this one is tricky, as it usually requires you to spend a lot. Very good power source, but expensive as heck. You can get it by spending 12k on ability scrolls (DO NOT DO THAT) or in various events for being 1st place. Imo just get them from cave key events- its safe way and you upgrade other stuff as well.

costumes- usually piece them for free and just finish with cp when you are close. Optionally ask on forum for specific costume.

If you are looking for power sources but as a mix of everything, pretty much what Qv said. Also explanation- lucky board is considered the best event for spending because of item value that you get. It loses some value after you finish medium thread/refine level.

Also personal biased opinion: dont buy medium refines/threads, get them over time and buy charms and advanced refines instead. Advanced threads are not worth buying(20cp:1 power ratio which is horrible). Myoboku gifts, mood scrolls and unboud runes are the best to obtain from wheels or great plates.

And dont forget- if you want to spend for power at events with redeemable part (mission dealer, konoha tree, lucky board, lucky stars etc.) always look first at the redeemable part and make sure you want something from there.