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.
Fermenting: Imperial IPA, Wee Heavy, Scottish 80/-
Souring/Funkifying: Brett C Old Ale, Lamebic, Flanders Red, Flanders Pale Ale, Oud Bruin, Session Kriek
Bottled: APA, American Amber, Scottish 60/-, Robust Porter, Brown Porter, English Cider
Bottle Cellar: Brett B Tripel, Brett C English Barleywine, Lamebic, Quadrupel, Tripel, Brett C Bitter, Wild Cider, Winter Warmer, Bourbon Barrel Sour Stout