I've never used lavender, but I'd be worried that (like a lot of aromatics) you might ruin it by boiling it. Then again, I don't know if that's the case for lavender. You might consider making a lavender "extract". Soak it in vodka to extract the aroma and flavor, and then use something like a graduated cylinder with an existing beer, adding small measured amounts at a time until you find the concentration you like.
Souring: '14 Brett C Old Ale, '15 Lambic, '14 Lambic, '14 Flemish Red, '15 Flemish Pale, '15 Oud Bruin, '15 Session Kriek
Cellar: '10 Brett B Tripel, '11 Lambic, '13 Brett C English Barleywine, '13 Quadrupel, '13 Sour Stout, '14 Brett C Bitter, '14 Spontaneously Fermented Cider, '15 Wee Heavy, '15 100% Brett B Red, '15 100% Brett L Kriek, '15 Bière de Garde