Flight
The Seattle-Tacoma International Airport (Sea-Tac) is the nearest major airport. It's about 2 and a half hours from Glacier. Vancouver International Airport is typically an hour and a half to 2 hours from Glacier, depending on Vancouver and border traffic. Bellingham International Airport will likely have less flight options at a bit of a higher price than the two airports above, but you'll land 50 minutes from Glacier.
Rental Car
You can look at sites like Priceline and Kayak to compare your options and find the best deal. Then go and book directly with the car rental company's site. The Priceline comparison site is linked right below.