What kind of film stock you use for practising is just a question of what you want to do with it. If you want to practise how to correctly expose film you'd better take reversal stock, because reversal stock has a smaller latitude. This means you can't just under or overexpose without even noticing because you will see if anything is under- or overexposed. With negative film, if it's not real bad, it'd be no problem to fix it - using reversal you can't fix it in the DI anymore. But if you just want to practise how to expose film I suggest you do that by taking stills - much easier and cheaper and enough for what you need.
If you wanna shoot a real film it depends on what look you wanna create - there can't be any rule on what film stock is best - there are many different because there are different cinematographers, tastes and tasks they have to suit. Most feature films today are shot on negative film, but just get some rolls of different film stocks to play with and try what you like or what you need. As far as I know there are only daylight reversal films available from Kodak but you can compensate for that with a filter - or you could shoot black and white, which gives you a very interesting look especially on reversal film, too. If you decide to shoot outside there'll be more than enough light to go with reversal film. So - just try some different stuff and experiment.
You can purchase film on eBay but got to be careful. Especially when using old color film stock of any sort it's quite likely that a color shift has been going on. If it's not too bad you can fix that in the DI. Also you have to be careful because the light sensitivity of the film decreases with time and you need to compensate for that - after a few years this can easily be 1-2 stops. So if you get film on eBay you should carefully test each roll. But if you just want to shoot some feet of 16mm - call Kodak and beg for a free roll or a recan - it's quite likely they'll give you one, and if not, at least a good discount. Most of the times Kodak people are real nice :wink: