The closest and Easiest Costco is about 3 miles north of HoJo's at the corner of Harbor and Orangethorpe.
So basically, leave the HoJo's parking lot and make a right, at the traffic light, make a right on Harbor, go up Harbor and OVER the 91 Freeway, and when you see the In-n-Out Burger, turn into the parking lot (right) just past the restaurant.
I love this area in general, there is also a lot of other places to check out than Costco. I would have a meal at In-n-Out, GREAT food, also there is a clearance/outlet store for the Disney Parks, a Target, and a Henry's Farmers Market (Grocery store), plus lots of other places