There are a lot of web sites that sell artificial myrtle topiary plants, but that would be expensive if you need to cut them up to use them. I did find two sources, but I don't know how close they will come to "bog" myrtle:

1.Silk http://www.pyob.com/herb_silk_bouquets.htm

2. Dried http://www.woodcreekdrieds.com/drieds.html