Are there any graphics of a small 'b' to indicate mBTC?
Unicode has some options.
If these are shown properly by your browser, the last two are lowercase:
Ƀ ᴃ ฿ B⃦ B⃦ B⃫ ␢ Ƅ
(Also note the second character is a small-caps version of the first.)
I wouldn't recommend you associate any of them with a particular portion of a bitcoin, though. In the future it could become cumbersome too.
For ease of use, how about just m฿ and u฿ or (properly) μ฿?
We may see the reverse become common worldwide for fiat currencies if they continue their devaluation.
That is, if it takes $3,200 to buy a mango, that'll be 3.2k dollars please.