You could try the bhw marketplace. I'd suggest contacting accounts in the niche you're interested in purchasing and asking if they have or know of any accounts for sale. Most accounts have an email in their bio where you can contact them.
Be sure to do your research before purchasing. Ask if the tweet activity was influenced by a tweet deck. Also ask for a couple of live tweets spread out across different times. Check the accounts who retweet them, and if the retweets on different tweets are made up of the same accounts or accounts that aren't actual people (i.e. comedy accounts or something) then the activity isn't legit.
There are also account buying/selling services out there, but all the legit ones I know of don't list accounts that are worth less than $500. Those services are the safest way to not get ripped off but depending on your budget it might not be applicable to you.
Best of luck!