Halfords, online, eBay loads of places.
Your doing it wrong mate. you need a piece of mdf 18 - 22mm think. If you've already made the holes in the shelf, you need to line the holes up and cut them out of the mdf with a jigsaw cutter. use about 6 flat head screws to hold the mdf to the shelf. Order some accoustic carpet and mega strong spray glue from cpc.co.uk. cut a piece of carpet bigger than the shelf, spray the shelf with the glue, smooth the carpet over the shelf making sure it doesn't dip into the holes. when it is dry, trim the edges and screw the speakers under the shelf. Easy
id just buy a proper shelf personally, however ive glued stuff before and its a messy affair, you cant stretch it as well compared to a staple gun either.
Look at the proper shelfs, they are stapled. if it ever sags you can remove the cloth easier if its stapled as well, and reuse it.