Here's what I can offer you for advice:
Go to WalMart and rescue a sick one.
JUST kidding - you'll actually be MUCH better off getting a nice healthy one. Things to look for are:
1) when you pick up the tiny cup they're in, make sure they swim around and don't just lay there
2) make sure there is NO cottony stuff on them whatsoever - that means they're HORRIBLY sick, also make sure they don't have any lesions, bumps or marks on them
3) don't get one with a swollen belly - that also means they're sick (swim bladder, constipation OR dropsy)
4) try to get one that doesn't have any rips, tears, curling, anything wrong with its fins (good luck finding one like this, but you never know, you might get lucky)
5) ideally, you should get one that is flaring in his cup at the guy in the cup next to him (99.999% of the time you won't ever find one like that, cuz they're so used to seeing another betta next to them they just don't even care anymore, or they're too bored and/or depressed to flare)
6) make sure their mouth and eyes look healthy - the mouth isn't swollen and the eyes aren't cloudy and/or protruding
7) make sure they don't look "puffy" or swollen in general - that can be a sign of dropsy (along with the swollen belly)
8) DO NOT buy one if it's scales are sticking out like a pinecone, either (if you see ANY of the signs above, you go tell someone in the store they need to take better care of their bettas, and be assertive about it) Okay, well you don't have to do that part, but I highly recommend it!
As far as things IN your tank, here's what to get/look for/do (for toys & stuff):
1) get them some silk (or real, your choice) plants - they love to lay in/on/under them
2) Get them some sort of hidey place/tunnel - they love to swim through things. BUT... make sure it passes the nylons test (or would). Run your hands all over the whole thing and make sure there's NOTHING that can scratch them - they're not real graceful creatures - they scratch themselves easily on things, and their fins get ripped. Also make sure whatever they swim through isn't very long, if they get confused they can get stuck and drown in there. They do need to come up for air, you know. Small hunks of PVC pipe work really well.
Ideal temp for them is about 78 degrees.
They don't necessarily deal very well with currents - if the tank they're going into has current, do what you can to slow it down. I'm finding that out the hard way with Amos.
Additional things to pick up:
1) Aquarium salt - don't use it unless there's an ailment or it needs a dip
2) Maracyn & Maracyn Two (I have these on hand just cuz I have 4 betta boys and somebody is ALWAYS ill)
3) BettaMax, if you can find it - every store in our area has discontinued it. Not sure why, but I've heard it works wonders.
DO NOT USE A NET WITH THEM. You'll rip and shred their fins. I always use a big ol' plastic solo cup to catch them in - just dip it into the water and the slight current that creates sucks them right into the cup, and viola! they're in. Make sure you cover that cup up, or you'll have a fish out of water.
ALSO... MAKE SURE if you're cleaning your tank near a garbage disposal, place a washcloth over the disposal opening, EVERY time, just in case. I've heard so many horror stores!! eek.
UNLESS you're a complete glutton for punishment like I am, then get a sick one an dtry to nurse it back to health. BUT be prepared for trying times and heartaches.
Congrats on your betta buying venture!!!
I can't wait to see what you get!
(sorry this is so looooooooooong)