A: For starters, get yourself the NYC Tennis Subway Map, which lists 172 public and private facilities in the five boroughs (send a SASE with 55 cents’ postage to USA Tennis ‘99 Promotion, 24-16 Queens Plaza South, Room 405, Long Island City, NY 11101). In Manhattan, there are several “tennis clubs” where membership is not required. The Midtown Tennis Club (341 Eighth Avenue; 989-8572), is one such facility. The club’s eight indoor and outdoor Har-Tru courts are open to the public and can be booked for either open (one one-hour slot) or seasonal (same day, same time for an entire season) sessions. Open times run from $25 to $68 during the summer and $36 to $72 during the winter depending on the day and time you want to play. Crosstown Tennis (14 West 31st Street; 947-5780) is another year-round facility, with four indoor rubberized hardcourts. Summer courts cost between $35 and $55 per hour but are slightly discounted for seasonal reservations (14 sessions in the summer and 36 in the winter). French Open fans should check out the pristine clay courts in Riverside Park. Use of the ten courts (with ten additional hard courts opening in August) is free with a Parks tennis permit ($5 per single session or $50 for the season; 360-8134). The only catch is that you have stand in line to sign up for your court the day you want to use it, a small price to pay to take that Boris Becker-in-his-heyday dive onto the soft red stuff.