Usually any party store will have decorations that are both pink and blue rather than on or another. If you're not able to find those then you can always buy. Mix of blue and pink.
The Party City here has some gender neutral decrorations that are yellow and green! They even have some that focus on the mom to be! Also, the dollar store has a few small banners and things... If you can't find much you might think about DIY-ing some decorations like banners or maybe some cute pomander balls...
I don't know of any "gender free decor sites" but back in the day before U/S they had to go gender free for every shower. I think they would just pick a theme that coudl be for a boy or a girl, (Noah's Ark, zoo, farm, jungle, beachy, or just "baby"). Most stores other pps have mentioned have decor that would serve for either/or.
Re: Leaving gender a surprise...
Try looking at Papermart, Target, and Walmart.
check out Etsy.com too.
you can do any combo of yellow, orange, green, white, gray, brown (+ mixes of pink and blue).