hobbycity.com is your friend. they have the cheapest batteries and those cheap batteries are better than even some of the expensive american name brands.
http://hobbycity.com/hobbyking/store..._40C_Lipo_Pack
i recommend that pack. i have the 20c version of that pack and it doesn't break a sweat in my brushless buggy, so the 40c pack will have no trouble pushing your muggy around. you could also get longer runtimes and possibly lower temps if you run 6s. the 6s version of that pack will work good. turnigy packs are the best packs on that website and i personally have 6 turnigy packs and they are by far my strongest batteries.
also, gear for about 35-40mph. you will get longer runtimes with a 6s pack geared for 40mph than you will with a 4s pack geared for 40mph. the muggy is a huge truck and shouldn't be geared for over 40mph with that motor.