So I love this game and actually don't mind the tiles especially when creating new boards, but I would like to know if there are any posters of the campaig maps available.

They definitely don't exist the way the skirmish maps do and for good reason. Nobody would pay $20+ for something they'll only play on once, maybe twice if they play the same campaign twice, and you'd need like 20 of those for a normal campaign.

Your only option might be a printable PDF, but you'd have to make it yourself for each map, and it would be a bit awkward since not all campaign are going to fit on a 8.5x11 or even 17x11.

A good tile organization system Will probably serve you well, and if you just sort the tiles you'll need for your next campaign mission ahead of time, it makes setup time much faster and is much easier than buying a single use mat or making and printing your own maps.

If (intentional) medium-res is fine, you can download the Vassal Imperial Assault Campaign module and its save file, run it, load the save file for maps, remove the initial groups, and save the graphics into file.

At the beginning I built quite a few maps on a photo-copier - had to cut and glue though due to only having A4-sized copier.