I use UPF53s on my 4Runner. I still have 17, yes 17, in stock. I bought them 6 at a time from an AC Delco jobber (parts warehouse) in Greenville, SC. I called them today and they are now completely out of all UPF filters. I bought my last 6 filters about 2 weeks ago. You might try looking for an AC Delco jobber in your local yellow pages or through the AC Delco site's search for a retailer (www.acdelco.com). They may still have a few. Great filters at a great price (I always paid $6.00 a piece).
Good luck,
Jack