These are the real deal, raw Japanese videos. Sometimes you can find them on eBay, sometimes you can find them at used sites, but the two best places are:
Distros (aka bootlegs)
These are mostly from fans who want to share Sera Myu with others, usually fan subs, and most of these sites say that they distro for the Japanese impaired, but still support them by offering links and not distributing EVERY musical. The fees they charge are usually only for the shipping and hardware costs (i.e. the cost of a blank DVD).
Sera-Myu Shop.Com - The subs are all $2 before shipping costs and the raw footage isn't priced... O_O
Usagi's Legend - If you purchase several at a time you get a discount. 5 for $20, 7 for $30 all before shipping.
Fan Subbing Productions
This is kind of like a bootleg and kind of not- the subbers put the product out for a small fee (sometimes) to give them a little boost for the time and effort they put into their fan sub scripts. Fan subs are not usually perfect (but then no subtitles or translation will ever be perfect) but are great for those who do not speak Japanese and don't want to miss anything.
I've only found one fan subbing production for Sera Myu at:
Cruel Angel Productions - Has a lot of subs to offer, and the quality is pretty good actually. Myu is offered for $5 per musical.
This is where you can download it for free on the internet for a reduced quality. It really compromises the sharpness of the musicals, and often errors can occur making it frustrating to watch a whole musical.
Sailor Music - has a lot of the Sera Myu songs from the CDs available to listen and download.
You Tube - has some of the musicals and many clips to view. Sometimes has errors loading, so it can be frustrating.