My suggestion would be to check out the thread on packers that's stickied at the top of this forum. That lists where to buy them and their rough costs. It all depends on how much you're willing to spend and what your priorities are (price, colour, 3-D balls, etc). Just have a look at each of them and choose, most of them, particularly the ones in the first post, are used by people on here, so you can ask any other questions if you're still undecided.
Personally I have the Mr Limpy XS, which is my first packer and that suits me fine. But just like cis penises vary so do our tastes in packers. Mr Limpy's are cheap though, so if you're not sure if packing is something you really want to do then it might be worth going for.
I don't know much about packing underwear, it all seems too expensive to be worth it for me, but my most expensive underwear cost £2.50, (so anything is expensive to me). If you're not sure on packing you can always make a harness really easily before splashing out on underwear, it might also feel more natural if you're a boxers kind of guy, I'd imagine packing underwear and boxers would make it feel more like the packer was clothes than your body.