You have to use a third party which fetches the data for you ... So at least 1 third party but likely 2 third parties.
The way it basically works is that someone has a proxy contract to which they can post information to send to other contracts. Your contract trusts this contract to receive information, and therefore implicitly trusts the source to accurately query the information from elsewhere and send it to you.
However, this can be commoditised (e.g oraclize.it), and you could use a quorum of many different proxy contracts.
The trouble is that ethereum operations and transactions (especially involving storage) are expensive, and that there are much cheaper ways of achieving distribution and/or trust for many use cases.
You have to use a third party which fetches the data for you ... So at least 1 third party but likely 2 third parties.
The way it basically works is that someone has a proxy contract to which they can post information to send to other contracts. Your contract trusts this contract to receive information, and therefore implicitly trusts the source to accurately query the information from elsewhere and send it to you.
However, this can be commoditised (e.g oraclize.it), and you could use a quorum of many different proxy contracts.
The trouble is that ethereum operations and transactions (especially involving storage) are expensive, and that there are much cheaper ways of achieving distribution and/or trust for many use cases.