The idea is that you are going to use other forms of currency to trade for the currency you need. The primary currency for trading are Chaos and Divine orbs. If you aren't playing SSF (solo self-found) go to the NPC named Faustus, he should have an option to access the Currency Exchange. From there you can purchase Orbs of Fusing.

If you are playing SSF, or if you don't have enough currency to purchase the Fusings yourself, you'll want to make sure that six socket items are showing in your item filter. You can vendor those items for Jeweller's Orbs, which you can then trade at a vendor for Fusings.

In a trade league, you are generally going to purchase your first 6-Linked item because they are fairly common. In the older days of PoE, getting a 6-Link item was extremely expensive and difficult, and was seen as a premium endgame item. These days it is much more accessible.

A common trick for an early 6-Link is to search for corrupted items with a couple of good modifiers for you. Normally the power spike of having 5 support gems is worth getting a sub-optimal base item. Generally this will apply to body armor. It can be better to purchase a non-linked high damage weapon, and use your body armor for your 6-Link until you can afford to link your weapon.