So I've spent a little bit of time trawling the interwebs for suitable 15mm scale Ork proxies and this is what I've found thus far:
Ground Zero Games
Possibly the largest selection of 15mm scale sci-fi figures in the UK at the present and they even produce some suitably hulking alien types that make excellent Ork proxies.
Vanguard have made a name for themselves in small scale gamers with their rather wonderful selection of Epic related sculpts but do also produce some really rather snazzy Boyz too in the form of Skinnerz.
Wartrukk (not painted by me!)
These Orkish types are more similar to the newer GW style of figures but look fab and showcase what one can do with 3d printing these days. I have to admit that I was unaware that they even made 15mm scale Trukks now too.
There's currently about a dozen packs in the range which includes some rather good warbikes for a Kult of Speed and are well worth a shot if you're looking for a more brutish Ork aesthetic.
SLAP Miniatures also produce a range of Ork proxies called Dorkz which have a somewhat more old school charm to them but are only available in a single pack which is a bit of a shame.
These chaps are a bit of a hybrid stylewise between classic Ork and newer 40k stylings and do have rather a lot of character with somewhat zany weapons and the odd xenos types the pack also includes.
Alternative Armies produce one pack of Ork proxies in their Space Viking range that I sculpted many moons ago. They're pretty crude by modern standards but they form the basis of my Badmoon warband:
I also converted some of the other Star Viking range into Ork heavy infantry for my own setting and they do the trick in a pinch!
They also produce some Malig Troops in their Ion Age range which look good too!
CP also have some interesting sculpts that again, while a bit crude, do have buckets of character that are worth a punt on.
I also converted some of the Human Adventurers to add to my current warband (as they were sculpted by me too, the style fits in with the Space Vikings from AA).
In related news, I've almost finished sculpting my warboss, Orko Fatbelly and will post some pics of his portly magnificence once I've got him finished and painted!
In the meantime, all the best!