With thousands of Republicans set to invade the city this summer, high-priced escorts and strippers are preparing for one grand old party. Agencies are flying in extra call girls from around the globe to meet the expected demand during the Aug. 30-Sept. 2 gathering at Madison Square Garden. "We have girls from London, Seattle, California, all coming in for that week," said a madam at a Manhattan escort service. "It's the week everyone wants to work.". . . Charging from $300 to upwards of $1,000 for an hour of companionship and a whole lot more, escorts said they can always count on conventioneers for big business.