My Carrier unit uses 16x20X4, I found them on Amazon for $27 for a Merv 2 pack. They were sold by a third party 'Complete Filtration Services', so they didn't qualify for free shipping. The 2 pack with UPS shipping was about $35 total and I received them in about a week. If you need a different size, just enter the dimensions in your Amazon search. I found many selections of the 19x20x4 size and all were under $30 for a package of two.
Here is a link [ame=http://www.amazon.com/16x20x4-MERV-Pleated-Filter-2-Pack/dp/B004E8UO5A/ref=sr_1_5?ie=UTF8&qid=1374844398&sr=8-5&keywords=16x20x4+MERV+8+Pleated+Air+Filter+%28 2-Pack%29]16x20x4 MERV 8 Pleated Air Filter (2-Pack) - Amazon.com[/ame]
