Is there an online generator to create a red/blue 3D image?

I haven’t tried it but perhaps this site is what you’re after?

These images are called anaglyphs and are bases on stereoscopic photography (think Viewmaster) where each eye is shown a slightly different image. You need to take two images and move the camera to the side about 4–5 inch (the distance between you eyes)for the second one. It’s also helpful to have an object in the foreground.
Just tried link above. Looks easy, but I keep getting a “communications error” when uploading my photos.

With that extra information from @kruger_d you can search Google for anaglyph generator and come up with a couple. Here’s another one. The example works so it should be usable. It seems you’ll need both images to be hosted online though.