If you don't find anything when you search for "tacticle weapons", you should try searching for "tactical excellence". Or you can go to their website, www.tacticalexcellence.com I had my USC conversion done by them. I registered my USC as an SBR, purchased an HK barrel, lower receiver, and stock, and then shipped them along with my USC lower off to be converted. I sold my original USC 16" barrel for $450.
Here is the break down of costs that I paid:
USC = $1100
UMP Lower = $250
UMP Stock = $225
UMP Barrel = $500
Tactical Excellence Conversion = $625
Sold USC Barrel = ($450)
3 UMP Mags = $180
UMP Flash hider = $130
4 Rails = $120
SBR Paperwork = $200 + 6.5 Months of waiting
So, I guess that brings the total to around $2900? If you are wanting to go the SBR route, it's a little tricker because you won't likely be seeing any more factory UMP barrels since the ATF decided to no longer allow importation of barrels. Instead you'll have to find someone to lop off the end of your barrel and install a flange from Shockwave if you want the UMP look (www.theshockwaveproject.com). The advantage of this conversion is that it's almost all original HK German parts, but it's much more expensive and you must register your gun as an SBR in order to use that many German parts.
Alternatively, you can spend alot less and get a conversion using US parts from Top Notch (www.top-notchparts.com). I believe their conversion costs:
USC = $1100
4 Rails = $120
UMP to Grease Gun Kit = $350
Conversion = $595
This conversion allows you to keep your 16" barrel if you want and doesn't force you to register your gun as an SBR since it uses enough US parts to keep you within the law (no more than 10 german parts). I have my suspicions that the US made sear that Top Notch uses will cause undue wear on your hammer, however, since it's a metal sear catching on a plastic hammer part. Tactical Excellence was making the same part and found that the additional wear was unacceptable.
Hope this information is helpful.
Allan