Arm’s reach 3-1 co sleeper bedding

Hi there, I’m a first time mommy. We have the Arm’s reach 3-1 co sleeper (not the mini). I’m 33 weeks and it’s occurred to me that the manufacturer’s mattress is very thin. I’m worried if baby boy will be comfortable. I’ve been searching for hours for sheets and mattress toppers but can to find anything to for this model. Does anyone have it? If so, any recommendations will be GREATLY appreciated.