I've been using one of these Graco bags for the past 16 months:http://cgi.ebay.co.uk/Graco-WATER-PROOF-changing-change-bag-MAT-for-pushchair_W0QQitemZ250270908345QQcmdZViewItem?hash=item250270908345&_trksid=p3286.m14.l1318
(btw just picked the listing at random from ebay to show the pic, don't know if it's the best price or even the reputation of seller)
It seems really good for space. For a 3/4 day out I usually have in it: a change of clothes for each of mine, about 8 cloth nappies, a couple of wraps, wipes etc. There's two bottle/drink holders at each end, a front & back pouch, which can easily each hold 4 children's books or other papers, plus a zipped pouch where I keep coin purse, keys etc. I've found it good value for money. The strap is nice & long, so is comfortable to hold on your shoulder, or wide enough to hang over a pushchair, etc.
I noticed when I just looked on ebay that there were more trendy coloured Graco bags, but not sure how the size compared with this one. Certainly I can fit a lot in it!