yarrow is commonly available herb and grows wild most places. wild weeds http://www.wildweeds.com/herbs/index.html#Top carries all 3 herbs.
The effects the gruit beer were way different than a hopped beer. It doesn't give you that sleepy, dull, depressed feeling that hopped beer does and it's stimulating, mildly hallucinogenic(more of a silly body feeling than a heavy visual high) and highly intoxicating. It's was very mentally stimulating.
For 5 Gallons
7 1/2 pounds of 70% wheat, and 30% barley malt extract
1/2 pound 30L crystal malt
6 1/2 grams Myrica gale
6 1/2 grams Ledum palustre
6 1/2 grams Yarrow
White Labs american hefewiezen yeast
Put the herbs in for a full hour wort boil