Ebay is probably the best bet.
There are some fantastic airsoft type guns there - the really cheap Japaneese import ones. You cen get them for a few dollars each, and they look as good as the higher end airsofts. I made a couple of modded weapons a while back - using these really cheap BB guns. As they are really low quality, they tend to stop functioning after a while - but that doesn't matter, as they are only props, and they still look really cool.
But the point of a modded gun like that is to do it yourself - it's half the fun; and besides, you're not likely to find many modded BB guns for sale (in comparison to normal ones).
To modify and MP5 like that would be a simple job. The stock has been replaced (possibly by something that looks like the stock from this ebay auction
). That's a simple cut and screw/glue job, just cut the parts off that you don't want, and fix the new stock in place.
As for the furniture around the front end of the barrell - to make my custom MP5 hybrid, I did it the good, old fashioned way. I took the gun apart, and cut away the bit between the magazine port, and the muzzle (the 'barrell' section, if you wil) - but doing this completely ruined the functionality of the BB gun - so don't do this if you want to keep it working. I then attached the muzzle to a piece of 1/2" PVC pipe, which was cut to the length that I wanted my new barrell section to be. This was going to be glued back onto the main section of the MP5 - so what I had so far was the ordinary MP5, with an altered stock, and a very weird looking front end, that wasn't attached yet.
The next step was to cover over the barrell section - I had found a piece of black plastic tubing, that had grooves cut into it, about every half centimeter or so, and it was just the right size for what I wanted. After searching for a while, I found a plastic 'cap' from something (it may well have been a juice bottle, come to think of it), that just fit over the end of the black plastic tube. I then cut out a hole big enough for the inner PVC pipe to fit through the cap. I glued this onto the end of the tube, and gave it a coat of black spray paint. Then I put the PVC pipe and muzzle into place, and glued them there (I had to pack the black tube with some paper to make it a snug fit, by the way). What this left me with was a nice new circular barrell section, with the muzzle on the front.
Once that was done, I set about attatching this new barrell section to the rest of the gun. This wasn't as easy as I had hoped - because the small surface area of the end of the tube meant that it wouldn't glue to easily. However, I finally managed to get it to stick with a little patience - and I strengthened the bond with some window sealant type stuff that was laying around in my garage.
So it was nearly finished now - I also changed the scope for one that I had taken off another BB gun a while back, and I attached two tiny little copper pipes (about 5-10mm in diameter), sprayed black, along the top and bottom of the new barrell section - just to add to it's looks. Later, I managed to get hold of a grip thing from my mate - so I attached this to the underside of the barrell too (which was a bit of a pain, because I had to cut away the copper pipe to get the grip to fit). The final thing I did was replace the magazine for the MP5 - I bought it from a mail-order BB gun ccompany. It was a similar type of magazine, just a little longer. It didn't fit properly - but with a couple of strips of black duct tape wrapped over the top, it stayed in place, but could be removed if neccesary.
So all in all, I'd spent very little - but I had a very cool mod MP5 at the end of it. Unfortunaley, I got rid of it a while back - traded it for something else.
However, I might see if I can track down any photos or screenshots I have of it - and post them here, so you can get a better understanding of what I'm talking about.
Hope this helps!