Hey. The best sites to use for getting parts are Digikey, Newark and Mouser. Do you know what you are doing? What ideas do you have? The 1590G made by Hammond is a die cast aluminium enclosure roughly the same size as the Hana. Another box I like is the 1550P as its nice and small. I know of a good site that has full kits and I have priced it out you actually save some money. I like to buy like 10 of each part at a time so I have yet to use them but my wife bought a kit and built a Raptor 120W regulated box. Nonameboxmods is the site. You can use the OKR or OKL chips. Alternatively you could go with a 10amp, 20amp or even a 40a Raptor chip. You have the SX chips, DNA etc etc. I personally like the Raptor chips the best but I would NEVER suggest it to a newb because its very different from say a Hana DNA40 which regulates the wattage no matter what resistance coil is on top. The Raptor chips output a constant voltage and then you do the math in relation to your coil and you get your wattage. I love the chip for its simplicity, lack of screen, sheer power and price. I have a custom 26650(dual) 200w 40a Raptor box coming to me and I currently have a 20a dual 18650. You can get a lot of good info from the modders forum. I got my battery sleds, enclosures, electronics and switches from Digikey. I got my 510s from FT cause Im cheap. I picked up some wire at Home Depot. Building a regulated box is more difficult then a basic mosfet box IMO. You may want to try a simple mosfet box. If you enjoy using mech mods I think a parallel dual battery mosfet box would suit you well. My friend has one (hes building me one, woot!!) and it hits like two mechs at once plus the battery life is really good.
I have seen some niiiiice DNA40 mods built in 1550P boxes with dual battery sleds. Sooo small for a dual cell box. If I liked the chip I would find someone to commission for a mod like that.