What I've done a few times before with success is building two starters... a 10*P wort starter (1.040) and a smaller 2*P YPD starter. The wort starter goes on the stir plate which allows the brett and sacc yeast to grow. The YPD starter just sits in a jar.
YPD = yeast hulls, peptides, and dextrose. For peptides you can use any protein source, I use whey protein. For each liter you would use 10g of yeast hulls, 10g of protein and 20g of dextrose (corn sugar). I find the yeast hulls in the health food section of my grocery store, sold in bulk cheap. For a 5 gallon batch 1L is plenty big enough, and I'd do a 2L wort starter leaving it on the plate for two full weeks to give the brett time to develop.