There are some high quality boots that have been making the rounds lately. They are available on sites like alibaba.com for $40.00 shipped and look just like the originals.
They are however, missing the security stripes found on *some* but not all originals.
These are known bootleg serials, originally posted over at n-g.com.
So, if you see Metal Slug 6 on ebay and it seems like too good a deal to be true it probably is.
MS6 3500510
SSVI 3000086
NGBC 1702636
KOV 0901669
DB 0701200
RF 1401163
GGI 1100342
NW 1702036
FOTNS 3400167
KOFXI 3200964
DFR 0601236