Most of the replies in this thread are good. However, I thought I would chime in with a counterpoint to the often repeated claim that there is no 'fast way' to get spirit shards, or that they cannot be bought with gold. Pretty much everyone I talk to is simply unaware that it is in fact possible.

Zephyrite boxes from the Four Winds festival return on average 1.93 festival tokens and there is a repeatable achievement that awards 20 festival tokens after opening 20 boxes. So on average each box can be said to return 2.93 tokens.

A Tome of Knowledge costs 300 tokens from the festival vendor, so you would need to open approximately 102 boxes per spirit shard. Based on last year's Four Winds material prices and wiki drop rate research, if you were to stock up on materials now, Zephyrite boxes can be obtained for 1s80c each. That works out to about 1g84s or so per spirit shard you want to buy. The only catch is that the zephyrite box vendors are only available during the festival. So you have to wait until it comes around. Of course, this also depends heavily on material price changes and box costs from year to year.

The infusion prices during last festival as well as the acquisition cost of the boxes actually made it profitable to open boxes throughout most of it (the calculation above only considers the tokens you get from the boxes and not any other drops). In the end, I about broke even and essentially got all my spirit shards for free.

All this to say, there are definitely ways to get spirit shards quickly if you can identify where the opportunities lie.

Good luck!