http://www.memoryexpress.com/ has several modles listed IN STOCK.
They don't come cheap:
AMD Athlon 64 X2 4200+ w/ 2x512K Cache (Retail Box - Now in Stock!!!) $699.95
SKU: 05945 Part No: ADA4200BVBOX
AMD Athlon 64 X2 4400+ w/ 2x1MB Cache (Retail Box - SOLD OUT!!!) $759.95
SKU: 05946 Part No: ADA4400CDBOX
AMD Athlon 64 X2 4600+ w/ 2x512K Cache (Retail Box - Now in Stock!!!) $1049.95
SKU: 05947 Part No: ADA4600BVBOX
AMD Athlon 64 X2 4800+ w/ 2x1MB Cache (Retail Box - Now in Stock!!!) $1349.95
SKU: 05948 Part No: ADA4800CDBOX
I should have my 4200 this week, i got it on hold till tuesday.
Here's a handy guide to which boards/BIOS revisions will work with the X2 chips.