There's a lot of misinformation in this thread so I'm going to jump in with what I've learned from my experience getting people followers using my two accounts (100K+ and 230k+).
First off, don't even entertain the idea of selling that on Fiverr. It's absurd that people would even suggest it. The people who sell Retweets to their "100k followers" on Fiverr rarely have any real followers (if they did they wouldn't be on Fiverr).
The big accounts won't buy that from you since they trade follower goals with other big accounts for just reciprocal services. They may tell you the price they charge for getting someone a certain amount of followers though. If that's all your looking for you can pm me and I can tell you what a few of them charge.
Honestly though, you might be better off just posting affiliate ads. Selling followers sucks for a few reasons:
1) Legitimate buyers are hard to come by. Not many people are interested in paying the kind of money that these services are worth.
2) When you do find a legitimate buyer (and one that's willing to pay you up front), you're usually facing a deadline to get them well over 2,000 followers and "the law of diminishing returns" definitely applies to getting someone followers. You have to start spamming your followers by retweeting and telling them to follow a certain account in order to meet a deadline, and after a while a single RT may only get the client ~50 followers since you've already spammed them so much. You're eventually going to have to start trading RTs with other big accounts, and while they're Retweeting your client you'll be Retweeting their ads (which isn't very good for the account growth).
If you don't want to do affiliate ads, another route you can go would be to make new accounts and build them up using your big accounts (and trading RTs, shoutouts, etc. with a network of other big accounts) and eventually sell it. It's so much less stressful than getting another account followers, and it can be more profitable if you find the right buyer.
I've done both of those successfully, so feel free to pm me if you have any questions.