Yeah, 15K is the max for the purchase miles program. (Costs just over $400, which is pricey but worth considering if you are very close to a larger award.)

If you could get your Marriott total up to 10K, you could redeem that for 2K BA miles. (I did this before to clean out Marriott points as I rarely stay there, and it posted within a week.)

That'd put you within 1K miles. Anywhere you need to fly on AA? (Just watch out for the L and Q fares.) Or maybe a quick trip from BWI to LAS on HP? (Fares are cheap on that route, but miles don't always credit so fast for HP flights. AA credits faster.)

Rent a car? 500 points for Avis.

500 points for a double-dip stay at a Hilton. (Trouble is, for me these don't always post so quickly, seems to be erratic.)

Idine (ecdining.idine.com) is good for picking up some miles, but they typically take 3 to 4 weeks to post.

I think you could launder some AA or UA miles through Diners Club (2 for 1 would be the conversion ratio), but I'm not an expert on DC.