The National Express ticket office at Heathrow's Central Bus Station has a desk that can sell through train tickets via RailAir services. I think the NX ticket offices at Terminal 5 also offer this.
Reading RailAir drivers do not take cash on the vehicles. They never used to issue tickets full stop, but recently moved to accepting contactless card payment on the vehicle.
For anyone not buying online (which gives a small discount, and the tickets are sold open-timed) tickets can be bought at the National Express counters inside Central Bus Station and Terminal 5, and at the RailAir lounge in Reading Station. The NX desks at Heathrow Airport do also sell through coach and rail tickets, although for many passengers it can work out cheaper to buy the coach ticket separately.