The first photo shows some of my well-worn Sanding Sticks, pieces of scrap wood with Aluminium Oxide sandpaper stuck to one (or both) sides to produce 'sandpaper files'. I first become aware of these when I was at school - my Technical Drawing teacher used to use simple versions to keep sharp points on the pencils. Later I was reading and article in Finescale Modeller about a modeller who carved airplanes from wood and used sanding sticks that were in fact old wooden rulers with different grades of sandpaper attached with double sided sellotape!
The Aluminium Oxide paper is from Wilko's, and sold in packs of different grades for a couple of pounds. One pack should last years. The smallest one is not wood but sandpaper stuck to an eraser, perfect for small and curved parts, that's the great thing about these tools, if you have a specific modelling problem, you can custom build a sanding stick to match!
One other option is to use a layering construction technique; Wood - Double-Sided Sellotape - Polystyrene (from frozen Pizza packaging) - Double-Sided Sellotape - Sandpaper, this gives a sanding stick that has the slight 'give' of the manicuring boards.
I would recommend these simple and inexpensive tools to all modellers. The largest is 200mm long, 40mm wide and 15mm thick. The one at the bottom of the picture has a very rough grade.
Photo two, shows some of the very wide selection of Nail Files or Emery Boards I use, which you can buy from chemists, take a whole variety and see which best suits you. My favourite are from Poundland, three for one pound and on the back they have small plastic, half beads, which can be used for making turrets or look-out domes.
The bright pink one is a standard no-nonsense plain emery board, while the Flexifile (still in its packaging) is purposely designed for plastic modellers.