MCI will give you 2 credit (one per month for 2 months) on signup. They also have a 4-credit offer that they can offer if they call you or mail it to you. Maybe you can get it by asking, maybe not.

Diners Club points can be redeemed for RR credits. IMO that's the highest value option that DC offers, aside from the double-credit redemption "sales" on BA miles.

You could book on-line and fly a mileage run on one of the $30 Anniversary fares. Two round-trips would take about 8 hours and give you 8 credits. Bring some reading.

As a last resort you could buy credits from someone who has DC points to sell you.

Check the SW web site for info on which hotels and car rentals earn half a credit each.