In short, nimh like to be kept charged, unlike nicad. Once you are done racing for the day, charge each set up for about 15 minutes and then discharge again before a recharge next time you race.
Inline cell configuration is best if your car accepts them, the simple reason being is they can then be discharged on a discharge tray and all 6 cells can be pulled down to 0.9v before recharging.
If you have to run stick packs, the bulb set will be OK until you can get something better, but dont leave the cells discharged.
Not in the usa so cant really advise on where to get cells from