http://www.johnlewis.com/john-lewis-the-basics-iron-on-hemming-tape/p230859314loads of listings for it, apparently Sainsbury & Tesco sell it, I had some that was a big piece not tape but most sewing shops should have a choice. If you cut out fabric letters etc and use this to stick them you can stitch round them a lot easier.
Main thing with fabric paint is keeping the fabric pulled tight and getting the spacing right as you only get one chance, been there- done the flag!!