Sort:  

Try poloniex, deposit BTC and buy them with them. You can withdraw to your wallet from there.

Which should I buy? XLM or STR?

Can you explain more about how they work together?

Please have a look here on the link, https://www.stellar.org/about/mandate/

~~~ embed:872497673986813952 apart from this I am waiting for few updates from them from the events on 13th, 14th and 22nd. twitter metadata:U3RlbGxhck9yZ3x8aHR0cHM6Ly90d2l0dGVyLmNvbS9TdGVsbGFyT3JnL3N0YXR1cy84NzI0OTc2NzM5ODY4MTM5NTIgYXBhcnQgZnJvbSB0aGlzIEkgYW0gd2FpdGluZyBmb3IgZmV3IHVwZGF0ZXMgZnJvbSB0aGVtIGZyb20gdGhlIGV2ZW50cyBvbiAxM3RoLCAxNHRoIGFuZCAyMm5kLnw= ~~~

StellarOrg Stellar tweeted @ 07 Jun 2017 - 16:57 UTC

We are excited to announce that Chinese exchange #Yuanbao is adding #lumens for CNY/XLM transactions, June 8th 3PM CT. #cryptocurrency

Disclaimer: I am just a bot trying to be helpful.