I've found just two websites that sell decent frames at reasonable prices and I'd highly recommend both of them if you just have to have a frame for your beaded masterpiece.
The first is that American bastion of antique needlework patterns and supplies:
Click on the image of the purse frames at the top of the page and scroll through the wonderful selection. The measurements given are for the overall size of the frame, not the inside dimensions from hinge to hinge.
The second is less well known, but no less wonderful:
Both sites have a pretty good selection and the best pricing I've been able to find. I've ordered from each. Again, both have quick shipping times and great customer service. If you just have to have a metal snap frame I'd go with one from either of these two merchants.
Another recent discovery is on Etsy. Not as many vintage looking frames, but several of the modern ones will look almost as good. Check out ThaiSupplies.etsy.com
But what if you just don't want a frame or can't afford one? Well, there are alternatives.
The drawstring method is an old standby. Actually there were quite a few vintage patterns that have little beaded tabs across the top to thread a cord through. Or a "header" can be crocheted at the top of the bag. Basically a decorative border done in crochet, a header has little holes in the pattern called "beading". Cords are threaded through the beading and often twisted out of the leftover silk thread used for the body of the bag.
Zippers are pretty easy to install in the lining before sewing into the purse. It makes a kind of floppy top, but a bit of boning or buckram between the lining and the purse can solve that problem.
Speaking of buckram, it can be shaped into all sorts of tops and is often the inner layer on those little white Japanese beaded purses from the 1950s and 1940s. The edges of cut buckram are sharp and will wear away your threads and lining so enclose them in seam binding if you choose this method.
As you can see, there are a few alternatives out there. Look at purses in thrift and antique shops to see how the problem was solved in the past. You'll be amazed at the variety of methods used!