Quick google turned up the following site: http://store.eastcoastwheels.com/wheelbuilding.html

I imagine that there are probably a few places that could do the lacing for you.

Edit:  More "do it yourself" sites courtesy google

http://www.dansmc.com/spokewheels.htm

http://www.geocities.com/spokeanwheel/

http://thekneeslider.com/archives/2006/10/17/lacing-a-spoked-motorcycle-wheel/

I used to ride a 10 speed & trued my rims all the time. During a crash my rear rim folded & I straightened it. It collapsed in a turn once, too. Experience will make dealing with spokes easier, but ultimately, it's always tedious. Having the right tools, like a place to set the rim up & spin it, watching for wobbles & runout, cuz you gotta get it so the rim is the same distance from the hub, all the way around, centered & straight. If someone handed a newbie a laced up rim, with each spoke just put in, not adjusted at all, it would likely take the better part of a day just to get it good & close. I would pay someone unless I just had loads of patience & was pinching pennies. I think the lacing of it would be kinda tuff, but not the hard part. IF you decide to do it, just build pair of vertical supports to hold the axle & take a piece of wire & screw it to the verticals, with the end close to the edge of the rim, use the lugs on the spokes to set the rim where you want it. Some would say you need a dial indicator, but I think it can be done well enough with a wire for a referrence, just set it closer as the wheel gets closer to right.

denim and i were thinking about a 21 inch rim awhile back. the best place to get any spoked wheel work done is buchannons. it's kinda pricey at around 300 bucks. might try an intruder wheel though. if ya can find one. if you get it done dont forget to post some pics.

buchannons does custom rims and spokes, just google it. they are the best out there.