This simplistic bold poster design and rendering should be easy to emulate. There are many more beautiful vintage circus posters, but these seem closest to my preference with their rich, warm, and vivid colours:
http://www.bigpostershop.com/page/BPS/CTGY/cir

I will make a reference gallery of my favourites soon.

This image has good curtain reference as well as nice design like countless others in the large collection (342) of circus posters in the Princeton University Library:
http://libweb5.princeton.edu/visual_materials/Circus/TC093.html

I will make a reference gallery of my favourites soon.

KJA Posters And Prints - Circus Posters (4 pages):
http://www.rainfall.com/posters/Circus/catalog1.htm

I will make a reference gallery of my favourites soon.

Another bold poster from this site of circus overload at
'Tesoros Del Ayer.com - Exposiciones - Recuerdos Del Circo':
http://www.tesorosdelayer.com/exposiciones/
EXPOSICION%20RECUERDOS%20DEL%20CIRCO/fotos.htm

I will make a reference gallery of my favourites soon.

A beautiful circus poster font named 'Zebrawood':
http://www.identifont.com/show?Y1
A couple more nice fonts are on this site named 'Revenue' and 'Rosewood'.

I like Madame Talbot's Victorian Lowbrow style of dark and eclectic imagery:
http://www.MadameTalbot.com/
Includeing sideshow posters:
http://www.madametalbot.com/pix/posters/sideshowposters.htm

I think Jeff Soto's darkly upbeat aesthetic is the closest look we're going for though I don't have many good examples to specifically reference for this project. Detailed, vibrant, bright, yet yellowed, rough around the edges and retro flavoured.
See a few more Jeff Soto images found on Google a year ago (he has many more now).
Jeff Soto has a website:
http://www.JeffSoto.com/

Mark Ryden's paintings will certainly be a great source of inspiration. While I don't plan to be as warped or cutsie nor will I reference his usually static compositions and brighter pastel colours I do like his high contrast, busy and rich secenes and antiquated rustic essence.
http://www.MarkRyden.com/

I will make a reference gallery of my favourites soon.

Scott Radke's marionettes are beautiful yet too macabre for our happy production. Dial down the distortion and crank up the colour and that would be close.
http://www.ScottRadke.com/

I will make a reference gallery of my favourites soon.

Hugh D'Andrade's terrific Burning Man poster features too cartoony a look for this production, particularily in the perspective warp, however doing it with live action people will bring it back to 'reality'. I'm not afraid of getting too wacky with people.
http://www.HughIllustration.com/
http://altpick.com/members.php?id=17226

I will make a reference gallery of my favourites soon.

I worked on this animated TV show opening for the Time Warp Trio when Guru Animation Studio was starting up in Toronto in 2000. In reference to our project I like the shading, lighting and texture as well as the contrast though I would go for more primary and secondary colours. The stars are terrific, the skewed (not warped) perspective is too extreeme for live action (way too wonky table and room, the earth's curvature, etc).

Click each image to see it larger.

If Kancho can build a cardboard jet then we certainly can build a rocket:
http://kancho.muncho.org/gal_f104.html