Actually our 'ancestors' in knitting worked in the round on humongous dpns; regular straight needles have only been around since the 19th century. You can use 8 or 10 dpns if you have that many in the same size and the sts will all fit on them.
Even if the circs you have 'aren't the right length' there's ways of dealing with that. Magic or single loop for hats or gloves or sleeves to make a small diameter piece on longer needles; if they need longer needles you may have 2 the same size that you can use. But most patterns can be changed easily, some, like cardigans, can be knit as is, the circ is just to hold a large number of sts, but if you have 2 sets of straight needles the same size, you can just trail them across the row.
You may or may not want to add an extra st to each edge that you sew up to account for the seam. For a bottom up sweater, just divide the CO sts in 2. It gets a little trickier to change a top down raglan into flat pieces but it can be done.
There's hundreds of nice patterns as Berocco.com and most of them are done in pieces and sewn. Lion brand also has a lot of flat knit ones, more than are done in the round. Check the other yarn websites too, they don't seemed to have jumped into circulars. And knitty.com has many nice straight needle patterns too.
sue- knitting heretic