All bolsters are NOT alike. There are round bolsters and rectangular bolsters. Some are soft, some are hard or stiff. So how do you decide what is the right bolster for you?
A bolster is used for a variety of poses. It helps to separate your pelvis and chest for more space in supine posses like bhadokonasana or supta virasana, provides a lift for your pelvis in viparrita karani, supports behind your knees in savasana or for your chest and spine in setu bhanda sarvangasana or reclined pranayama.
A bolster is not just for restoratives but can be used to access more active poses for longer holds such as lying back on a bolster on a chair in urdhva danurasana. A bolster is especially appreciated for head or chest/abdominal support in forward bending poses during menstruation.
So which bolster to buy? The bolsters we used in SHIZEN Yoga Studio (and now in @Yoga Studio) are from YogaWorks Japan and Bhekha Yoga in the states. However there are a variety of companies offering bolsters now and a variety of options found on amazon, rakuten, and Yahoo shopping. Although there seem to be many cheap options on the internet, I have found when it comes to a bolster you get what you pay for. Cotton or wool filling blends are not cheap but may offer better support than purely synthetic options.
When selecting a bolster it should be firm, not stiff. It needs to give a little bit but still offer support. It should not be too wide to lie over on width wise, or too short lengthwise. If you have longer spine, a short narrow bolster may be unsatisfactory for your support needs and body proportion.
Consider what poses you plan to do with a bolster and choose the bolster size and shape that would suit your body type best in those poses. A bolster is a worthy investment that will support you for many years to come. As word of mouth is a great resource please let us know what bolster you have and your critique about it in the comment.