What I am wondering is how some people regularly get upvotes from 25 reputation accounts with huge Steem Power (50+). Any idea?

These accounts could be selling votes to minnowbooster and smartmarket. They could also be just people who are holding STEEM mostly as an investment. They could be sock puppet accounts making a circle-jerk. There are many possibilities.

As for using bots, Try Smartsteem or Minnowbooster They guarantee an upvote worth more than the SBD sent (as long as STEEM price doesn't fall down). You can also follow the link and register to sell your upvotes. Personally I've set it to sell vote when my VP is above 98% but you can choose whatever you like.