small angular bearings and bearing contact angles

On the topic of angular bearings, does anybody know of a supplier for 0.250"
ID or 5/16" ID angular bearings? I've found 1/8" ID, than 10MM ID and up,
but nothing in between.

Also, in reference to the contact angle of angular bearings used for
leadscrews, should I just split the difference and go with the 45' units? Or
is there a benefit to the 15' and/or 25' bearings?

Andrey