The percentage chance of any particular Chansey holding a Lucky Egg is low enough that you'll probably need to get more than 7 Chanseys to the item, so just keep trying (if you haven't gotten one by now). Unfortunately, I don't know of an especially easy way to get a Lucky Egg in GSC. It may be easier to just train your Pokemon more slowly and not worry about it.
If you look on the route near the house where you can trade a Chansey for Aerodactyl, I think you can find Chansey at level 28 (whereas the other Pokemon are lower-leveled). Thus, with a Pokemon at the right level (higher than the other Pokemon, but not higher than Chansey), you should be able to increase your Chansey encounter rate using repels. That won't improve the Lucky Egg rate, but at least it should reduce the number of useless encounters.