As rfusca answered, borrowlenses and lensrentals are probably the most popular. $100 per day for a Canon 5D Mk II LOCALLY is not expensive to me. That is quite reasonable. Lensrentals.com has a minimum of a 4-day rental period, but it will also require you to pay for shipping, but in the end the daily rate will be less then if you rented locally at $100/per day.
It sounds expensive to rent, but renting lenses is a great idea and almost always worth the expense in my opinion. If you are not familiar with a focal length, renting it first is really the only way you are going to be certain that is what you need. If you already have a Canon body I would just rent the lens, and just buy the new body. If you are upgrading from an older body, you will almost certainly find value in the 5D Mk II, being Canon's most popular professional model.
As a side note, my local shop in Minneapolis charges $150/day to rent the 5D MK II, and I use them all the time for lens rentals and find the prices very reasonable.