The ones coming from London especially are tricky because you have to cover international airfare in addition to their fees. Alfie, though, for the time being is in Los Angeles, which is Good!

That is good! You may also have luck with one or a few of the following Harry Potter actors, all of whom may be located or filming in the U.S./Canada:

• Dohmnall Gleeson/ (filming "The Revenant" in Burnaby, British Columbia, Canada until 3/25/15) (located in New York City, New York?)
• Christian Coulson/Tom Riddle in COS (located in New York City, New York)
• Gary Oldman/Sirius Black (located in Los Feliz, California)
• Luke Youngblood/Lee Jordan (located in Beverly Hills, Californa)
• Scarlett Byrne/Pansy Parkinson (located in Los Angeles, California)
• Freddie Stroma/Cormac McLaggen (located in Los Angeles, California?)
• Bonnie Wright/Ginny Weasley (located in Los Angeles, California?)
• Evanna Lynch/Luna Lovegood (located in Beverly Hills, California?)
• Robert Pattinson/Cedric Diggory (located in Beverly Hills, California?)
• Michael Gambon/Albus Dumbledore (located in Beverly Hills, California?)

? = two different locations listed

I decided to suggest the list above because the actors who now live (or work) in the United States, or Canada, may prove more cost-effective in terms of paying for their travel to and from Leviosa. Additionally, those HP actors who may be currently located in California are much closer to Vegas (and Nevada) than those located in the UK or Ireland. Michael Gambon also may or may not be an option, as he recently retired from acting due to short-term memory loss, thus (presumably) freeing up his schedule.

Most locations are taken from fan mail addresses of these actors/actresses, and may or may not be currently correct. Personally, the only one I've confirmed relatively recently is Christian Coulson's fan mail address and location. Additionally, Robert Pattinson may no longer be doing conventions, though he has done "Twilight" conventions in the past (2012 and before). Evanna Lynch may be a good option, as she also attend "A Celebration of Harry Potter" in January/February not too long ago.

Personally, I would absolutely love to see Naomi Novik visit, even if she has nothing to do with Harry Potter. /u/Annul, who visited Infinitus 2010, informed me that Leviosa may have some of the same coordinators (?). Novik is one of my favorite authors, and her books (the Temeraire series) was optioned by Peter Jackson (The Hobbit, LOTR) for a film or TV series.

Other options and guests could also possibly include the Hillywood Show, a group headed by Hannah and Hilly Hindi that attended AccioCon in 2008 (and have since done several viral Harry Potter YouTube parodies), wizard rock bands, and cast members of A Very Potter Musical (and/or its sequels). Leviosa seems to be focused on writing and YA authors, so I'm not sure if these choices may or may not be ideal, but I thought I'd suggest them as potential options nonetheless.