If you are close to the north end (Wet n'Wild) then it's the one along Sand Lake Road at the junction of John Young Parkway. Take Sand Lake Road for about 10-15 mins from I-Drive and it's on your right as you cross John Young Parkway.
If you are at the southern end (say around Seaworld) then I'd suggest the new one SR535. Go the southern end of I-Drive, turn right (west) onto SR 536 (runs east/west) and then left (south) at the next junction with SR 535 (runs north/south). You'll find a huge Walmart about 2 miles down on your right.