Do you need anything special for Restorative Yoga or Deep Relaxation?
It's not necessary to go out and buy a bunch of stuff for restorative poses: You can use your own blankets and pillows. In fact, in most of the poses I demonstrate on this site I am using my sofa pillows and some spare blankets.
Bheka Rectangular 100% Cotton Yoga Bolster. I haven't tried it myself but it looks heavenly. They say it is big, soft and durable; ideal for relaxation and restorative practices because it is softer than most other bolsters. You can really settle into it but its large size makes it supportive too. This bolster and lots of other stuff are available at www.YogaLifestyle.com.
Of course there's lots of places to get props like Yoga Direct, The Yoga Warehouse, Yoga Accessories, Yoga Props, and even on Amazon, to name a few.
I'm a big proponent of using what you have on hand for your practice, but if you don't have what you need to get comfortable I would consider investing in a bolster. You have to be comfortable when you practice Restorative Yoga and Deep Relaxation.