Virgin Mobile USA is expanding the retail footprint for Broadband2Go, its 3G nationwide wireless Internet service with no annual contract.

Retailing at $99.99, Broadband2Go can now be purchased directly from Virgin Mobile USA’s Web site, as well as a more than 1,000 Best Buy Mobile stores, more than 4,300 Radio Shack stores, Sam’s Club kiosks via Radio Shack, Walmart.com and Target.com as of Dec. 1.

Extending its pay-as-you-go mantra, Virgin Mobile says megabytes can be added as often as needed with credit/debit cards or Virgin Mobile Top-Up cards. Top-Up cards can be used to purchase data usage from 100MB to 1 gigabyte on the pay-as-you-go model.

A new Broadband2Go-branded Top-Up card for $20, which customers may use to purchase 250MB good for 30 days, translates roughly into 12 hours of Web browsing. Other data plans are available at $10, $40 and $60. The MC760 Broadband2Go device was developed in conjunction with Novatel Wireless.

Virgin Mobile says its prepaid competitors either price their services above Virgin’s, as is the case with Verizon, or its competitors’ speeds and bandwidth are below what Virgin offers, which it says is the case with Cricket.

During Sprint’s latest quarterly conference call with analysts, Sprint CEO Dan Hesse said the company is on track to close its acquisition of Virgin Mobile in the fourth quarter, with the goal of strengthening its position in the growing prepaid market. Sprint has said it plans to continue using the Virgin brand alongside its Boost Mobile prepaid offering.