No broadband ISP will support anything like that except possibly a smaller DSL provider who wouldn't have to implement it on a network across the nation. Too much work and cost to implement for the very few people that desire it, and they'd rather upgrade you to a faster speed with a business account. I've seen nothing about Linksys or D-Link creating such an item either, as their product lines are designed for low-cost usage. High-end equipment such as from Cisco can easily load-balance traffic but that's designed for T1's and higher; I doubt their DSL product lines can do it. Netopia makes routers which can bond two ports but again, your ISP will have to support it (at the very least, they have to route your IP block over both connections so the traffic will load-balance).
The term you want is also "bonding" of connections.
What do you need "ASAP" exactly? If you so desperately need bandwidth, you should be considering something besides a residential line. If 1.5Mbps downloads or 256k to 500k upload speeds aren't enough, you're probably doing something you shouldn't be (violating terms of service or copyright law immensely).