Your ROI (=Return On Investment) does not depend on the amount of Hive Power you have because ROI is the ratio of your return and your investment.
You can buy HIVE at least from:
Blocktrades - blocktrades.us,
Ionomy - ionomy.io,
Bittrex - global.bittrex.com
and Probit - ProBit Globa (http://probit.com)l
Huobi is looking into adding HIVE as is Binance.
The Hive blockchain can be accessed using the following web front ends among others: