According to the following URL, the 3661 has indeed reached end-of-sales, meaning that you can no more order it. It is still supported though. Please ask your SE for alternate product.
http://www.cisco.com/en/US/partner/products/hw/routers/ps274/products_eol_models.html
Hope this helps,
Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México