Your probably referring to this H bridge diagram, http://s.eeweb.com/members/von_wong/answers/1335122304-h-bridge.gif Or possibly a simple H bridge chip, http://www.elektroda.pl/rtvforum/files-rtvforum/ckt_mtr_1826.jpg
If your motor take less than 1A you can use this http://www.farnell.com/datasheets/39120.pdf. It comes in an SO-8 package, so you will need an SMD adapter board to use it unless you are designing your own PCB.There are some low-cost H-bridges around based on the L293D and L298N bipolar H-bridge chips, but they have a voltage drop of around 2V. So you would need a power supply providing around 8V for your 6V motor if you use one of those.Another solution is to use a single mosfet to control the motor on/off and speed, and a DPDT relay to reverse it.
If your following the first link I gave you then, tip 41, should be fine. But looking at the data sheet, its made for high voltage applications. Kinda overkill for your project. You may just want to get some 2n3904 or 2n3906 NPN transistors from RadioShack, or simply a H-bridge DC Driver IC.
No diode needed, but if you are having problems finding the parts, go with the other given alternative from dc42
I think he means my reply #5 in this thread.
If you update your profile with the country you live in, then we might be able to suggest suppliers.