I reckon you can buy the stuff you'd need from a shop.. It depends though what we are up to !

There's load balancing to give us greater bandwidth, or there's multiplexing to give us greater speed - the second one is straight out of the window as a choice, as that'd depend on having the ISPs at the other end co-operate & if they'll do that, buy one faster line & split that up/share instead!

If we are doing load balancing it's best if we do that at the router level or at least outside our box, as multiple NICs present or not, windows can only talk to one connection at once (well tcp/ip can only use one gateway at once) so we want something like this ;- netgear prosafe range ($200 or less I'd have thought) ..then you'll have to get all Heath Robinson about how you wire it up, depending on what you've already got..

& btw by way of illustration as I know folks get confus-ed about these two phrases ..
..Dual Wan Routers dont double your 'speed' They increase your bandwidth. If you DL a 2MB file it will not DL any faster with two WANs, but if you DL 2 such files they will each DL at the same time via balancing....