Hi looking to have some custom decals,
does anybody know how I can do that for cheap?
I need to get them done ASAP.

Any ideas?

RCMW usually carries adverts. Check back a few copies. There are three or four people in the UK doing it.

try here:
http://www.tailormadedecals.com/xtc/

If you have a good color printer you can do them yoursef, ACT Aircolor carries the carrier film in white and transparent.
Beware that you cannot print white, so if you need white letters on colored underground you need to find someone with a microdry printer.
I've done markings for a couple of planes, they all came out great on my Epson 760. Should not use Lexmark or HP or similar on those graphics, they come out quite dizzly...they do not have a special mode for these types of film.