i'm trying to buy last months (february) collection bag. it still shows up on the main page of the cash shop (under popular) but i can't buy it from the main page, and i cna't seem to find it anywhere else in the cash shop.

is it not available anymore since it's march now?

Each monthly collection is only available during that month. It will show up on the main banner as long as it has a higher number sold than enough other cash shop items to bump it off.

also, the February 2013 DP had some really cool stuff in it. Usually a DP is pretty expensive to me, so I only get one item and only if it's groovy. I bought three. I looked at the March 2013 bag and went: meh-beautifully rendered, but do not want.

Usually unopened DP bags and their items deflate over time (a year or so). I'm not so sure that this one will. There's a chance that your taste is different from most users. Check and see if the item you want is available cheaper, and check it again in 3 months or so. You may be able to snag a deal as fashions change/people change their mind/finally get around to pricing their shops.