Depends on what you want to do with it, as the carrier account is linked to one single player.
If you just want to use one to get from A to B, hook up with a carrier owner (there's a Discord for that) and seee if someone goes in your direction. There are more or less regular carrier services between the Bubble and Colonia, for example. Tritium donations are always accepted.
If you want one to store your fleet, then you need to find a carrier owner with the same travel plans as you have, who also has the (optional module) shipyard equipped.
Otherwise, carriers are pretty much everywhere in the Galaxy now, a lot of them have services available for everyone/clean pilots only/squadron only/friends only. The latter two may be combined. You may find them in CGs (Community Goals, see https://forums.frontier.co.uk/forums/elite-dangerous-community-goals/ ), especially in trade CGs when they're supplying/shipping the CG trade goods. Check the system chat, and you should be able to get an idea which player groups are currently active and whether (and under which conditions) they will accept freelancers to help unloading (in the CG system) or loading (in the supply systems) the carriers.
For example, a carrier owner may decide to jump to a supply system, i.e. a system that has a high amount of the CG's wanted cargo available at a low price. Where the station is in this system is irrelevant - the carrier can (if parking slots are available - those are limited) park right next to the station. The owner can then put up a buy order for, say, 20kt of CG stuff at 90% galactic average price. Since you can buy from this station at e.g. 30% galactic average, you can make some profit from buying at the station and selling to the carrier. When filled up, the carrier owner will usually announce the jump to the CG system. Dock on the carrier, and he'll take you along. Once you're in the CG system, the carrier owner can then set a sell order for their cargo at 110% average, while you can sell that stuff to the CG station at 400% galactic average, again making a profit, as well as the carrier owner.