Unfortunately the cards were discontinued years ago, so there is pretty much no chance of finding them in your average store. Our only options for Digi-Battle cards would therefore be from those halcyon days years past (xd), or ebay...possibly garage sales or some thrift store. I've seen a whole myriad of decks and random cards over at ebay, but alas, I haven't bought any xD
In order to practice some of the decks I've made (since I didn't have the cards and didn't have money to buy the ones I needed) I ended up finding scans online and printing them off on card stock, lol. Not the best option, but if you need a more convenient way to practice your decks it's the simplest and most useful thing you could do (short of creating two accounts for yourself and practicing that way...which is frowned upon *cough**cough*).
Collecting the cards, however, has become a luxury for us Digimon fans. Same with collecting those old Tamagotchis. Short in supply, monster prices. Although, maybe that isn't such a great analogy, 'cause the prices for cards aren't too shabby. Some individual "rare" cards go for a couple dollars (up to being totally ridiculous), but the bulk bunches and some booster packs aren't too bad. Japanese versions of the cards, however, tend to stack up in the higher price ranges pretty dern fast. To the best of my knowledge, what you're looking for are the "alpha" stages of the JP cards--the ones with the cool... swirly...symbol...thing or whatever on the back. The newer card game in JP may or may not have changed that design.
Guess I went off on a rant <.<
Short answer: for all intents and purposes, it is impossible to buy Digimon cards locally. I'd recommend ebay or, if you can't buy from ebay, a good printer and some cardstock xd
Best of luck.
Edit: I take it all back. The cards on ebay are ridiculously expensive. Why anyone in there right mind would buy three cards for $50.00 is beyond me <.< It's all very strange, because just a couple months ago, there were lots of 50 cards for only $5.00 or something...
Anyway, my new recommendation: find yourself a printer and print off some scans xD