You don't need special software to print a poster actually...any software will do even GIMP or what have you...
See when you install the driver for the printer you get everything you need, you have to go into Printer Properties at time of printing. I usually have to change my settings from CM to Inches. The cruxt is having your file at 300 - 600 DPI (dots per inch - 600 is a lot but it's what you need to pull a good print). So you have to have a HUGE beautiful file that's ready for printing and a big plotter / printer to print from. I have a big HP plotter that was about $2,000. Each ink cart costs me about $40 each. Generics are about $20. I am now stocking generics encase I run out because it takes forever to get my ink from B&H.
You have to be good with machines in a way because the printer is pretty finicky and it took me a couple of months to learn all its eccentric ways so we would have a good relationship.
Some of my files I max out at 2gb in Photoshop. Poster files are B I G - that's why it's hard to find good posters for free online.
I do requests if you need something large printed. Sometimes people have requests for box art, or a page from a players guide or Nintendo Power Magazine....they send me the item in the mail, I scan it, make the poster file, print it and ship both their original and the new poster back. If it's something I can resell I don't even charge. ( I don't print art from Deviant Art etc. that doesn't belong to me, original modern illustrations, or for games that are not retro like Assassin's Creed - I just do vintage.....if you have permission from the artist then that would be okay, but I don't want to step on someone's toes or violate copyright any worse then I am.)
I love to talk shop about printing, particularly posters since it's what I do ( that and game design - but game work takes a long time before you can show what you've done! But I'm making sprites and illustrations for a game that will be out in 2014 at the moment...).
I would really like to hear what posters everyone would like to see restored or created fresh. I have a game library and try think what I want in it and I ask my friends etc. but would love to have more ideas!!!
And a big THANK YOU to Retromags because for someone like me who restores and researches it's an amazing assett. If I can ever be of help let me know.