HI there, welcome to the forums! Now, I really suck at explaining things, but I'll try to help you the best I can.
When somebody is looking for a villager, they'll post here:
http://www.belltreeforums.com/showthread.php?112564-The-Official-Looking-for-Trading-Buying-Thread. Whenever you're looking for a villager, always post there. Making a separate thread somewhere else will always result in that thread being locked.
If somebody who has Mitzi sees your post, they might contact you, by either leaving you a message on your profile (otherwise known as a VM, a visitor message), or they'll send you a private message, which you'll receive in your inbox. Now, if you're lucky, the person might give you Mitzi for free. But usually people on here either look for IGB (in-game bells) or TBT (this forums currency, which you earn by posting and being active on the forums) in exchange. I can't really give you an estimate as to how much she'd cost, however.
Another way to inform people you're looking for Mitzi is to put it in your signature (the place where you currently have 'dreams' written). That way, when you post around the forums, doing whatever, people will see your signature and become aware that you're looking for Mitzi.
Also, the link I provided for you is posted in the villager trading plaza. There, you'll find a bunch of cycling threads. Cycling threads are from people who have copies of acnl with the full intention of moving villagers in and out. Some people give villagers away for free in their cycling thread, but others have set prices, so if they ever come across Mitzi, they'd already have a price set out for her, so you could get an estimate on to how much she'd cost. Also, most cycling threads allow lurkers. Which means if you left a comment on one of the cycling threads, asking to lurk for Mitzi, they'd add you to a list and if she were to ever move in, you would be sent a message.
So, long story short, the villager trading plaza is the best place to look when looking for Mitzi. But as I already mentioned, stick to asking to lurk for her in the cycling threads, or post about her in the thread I linked you to. Posting a separate thread of your own asking will her will only result in the thread being locked.
Also, if you check out a cycling thread that has set prices for their villagers, you may see the prices listed as this:
Tier 1: -insert price here-
Tier 2: -insert price here-
Tier 3: -insert price here-
Tier 4: -insert price here-
Tier 5: -insert price here- (although typically these guys are given away for free)
So, obviously the tiers won't actually have -insert price here- next to them. The tiers come from this guide:
http://www.belltreeforums.com/showthread.php?354714-Villager-Popularity-List It's basically a popularity guide. Tier 1s are the most popular, with tier 5s being the most unpopular. It's based off of how often a villager is looked for, usually. Mitzi is currently in Tier 2, which means if a cycling thread has a set price for tier 2 villagers, then that's what you're looking at. But, keep in mind that prices may vary from cycling thread to cycling thread. But really, your best bet is to just look at any and all of the active cycling threads there that give villagers away for free. Also, some people who don't own cycling threads and aren't apart of them may post about villagers moving out, so keep an eye out for that, as someone may post about Mitzi one day.
And also, when the time comes for you to get Mitzi, in order to get her to move into your town, you must have less than 10 villagers. Nobody can be moving out when you go to get her, and nobody can be moving in. When you go to get her, go into her house. She should be in boxes. She can only be convinced to move in when she's moving, and in boxes. Talk to her. She'll talk about how she's moving, and how she has nowhere to go. If all goes well, an option should pop up where you can tell her to move into your town. And if everything's good, she'll agree and her plot will appear in your town the next day.
From this point, not sure how much more information I can tell you, considering this post is already extremely long. But I hope I helped, and if you have any more questions, feel free to ask c:
