I get a lot of questions about framing & mounting options -- and there are so many choices. But here are several options you might consider when framing your artwork. See the example below that illustrates how I like to frame these prints.
First off, the prints are 13" x 19" - not a truly standard size, but this is the size the original WPA posters were printed. That said, there are plenty of options! Just make sure the frame is either for a 13" x 19" print, or that the matte board has a cut out for 13" x 19" pictures.
$10 - $50
On the less expensive side, you could get some minimalist frames like these "Format Frames" at B&H Photo. The frame fits the print perfectly, and there is no matte around the print.
This Studio Decor frame from Michaels has an inset perfectly sized for the posters. It also has a nice white matte board and a decent composite frame. It usually sells for about $40, but is often on sale for about $22, or you can use a coupon code for a better deal. Follow the link below the photo to see the frame on their site. Make sure to choose the one for 13" x 19" (finished size 16" x 24").
Amazon offers a wide variety of choices and price points.
$50 - $99
BH Photo offers a complete framing kit to give you an 18" x 24" finished frame.Click here to see the 18" x 24" Framing Kit.
Next up would be a more DIY route. I would try to find a 18″ x 24″ frame at Michaels, Target, Hobby Lobby, etc., for framing a poster. Hopefully one that's on sale -- or even on clearance. Then, just have a custom matte board cut to fit the poster and the frame. I have the inside of the matte cut to 12-3/4" x 18-3/4", which is enough to hold the print down, but still display the number, signature and date. You could add a second matte for a more substantial look, and I would have that cut at 12-1/4" x 18-1/4". With one or two mattes, you can choose the color(s) to match the print, the frame, and or your decor. I've found Hobby Lobby to be the least expensive place to have matte boards cut.
Then you just need a little double-sided tape, a ruler and a little patience to mount the poster to the backer board, over lay the matte board(s). Then, slip it back in the frame and tighten down the tabs in the back to hold it all together. You might also want to buy a wire hanging kit at the craft or hardware store. Usually with two screws, you can attach the wire so it's ready to hang! Depending on your frame choice and number of matte boards, it's possible to go this route for $50 - $99.