Have you got the roof rails installed? You'll need those to fit any of the cross bars too first of all.
I've got the Fiat cross bars for mine, basically did a deal when we bought the car. I'm sure they probably are made by Thule (or similar) and just badged as 500 as they look remarkably similar to Thule fixings I've used in the past.
For a roof box I bought one in Halfords via their flash sale last year, it's their own model but looks smart and has survived being kept outside over the last year or so.
Thule are the top level but you will pay top dollar for them, I'd suggest a wander around your local Halfords or other retailer and see what looks good to you.
Just remember you need the rails, I don't believe there are any other methods for fitting cross bars to the 500X.