What tyre widths I can use? Just yesterday I noticed that my tyres soon need to be changed but my tyre size is  27.5 x 2.8 But the closest sizes I can find are 2.35, 2.4, 2.1. Those were the closest sizes I could find in store (because I don't buy stuff overseas). What should I do or which size will fit?
'27.5' refers to the rim diameter, It is the same diameter as '650b' or ISO 584. The inch measurement tends to be used for MTB wheels, '650b' for road wheels. You obviously have to fit tires for the correct rim diameter.

'2.8' is the approximate tire width when mounted on a suitable width rim.

The inner width of your rims (distance between flanges) determines what range of tire width you can use. There are a few tables available on the internet that give general tire width ranges for different inner rim widths, but none I found cover the plus size range.

You can probably use tires that are narrower than 2.8". To make sure, measure the inner width of your rims in millimeters (a ruler is accurate enough), You should be able to find the rim width range that any model of tire is compatible with on the manufacturer's website.

An approximate rule for minimum tire width is no narrower than the outer width of the rim, preferably a little wider.