I'm doing the same thing. We have some wonderful landscapes here and I want to do some black & whites for our walls. I'm going to have them enlarged at shutterfly or kodak.com. Everything I've had done from both those sites has been pristine and good quality. I also like that when you select your options, they will tell you upfront if your photo doesn't have the resolution required for what you want to do. Pricing is similar between the two companies. I think one offers discounts through ebates and they also send out coupons which is helpful too.

You might look into using a local to you photo lab for something that size. You can look in the yellow pages or google for professional photo labs in your area. Then you might be able to talk face to face with someone about your project.

I'm a professional photographer (although I've been out of the business for a few years homeschooling my young'uns--fixing to jump back in.) Anyway, most of my work was larger wall portraits. I used a professional lab in another state at that time. A quick glance in the back of a photo magazine at Barnes and Noble might yield some info, too.
If you want specific info about the pro labs I used, drop me an email.

Good luck!

I have had blown ups made at Kinkos and they do a nice job with a variety of backing and paper options.

Thank you very much for your replies! I do have a sort of local bricks and mortar store that I forgot about. They do a lot of stuff.

I want a really big 'something' picture for in our DR.

I've gone as big as 18" X 24". $16 @ Walgreens online. When it's done, they call you. It was on kodak paper and they did an excellent job!