Two are usually recommended:
Camouflage Adventure
#37, New Street A (behind U Care pharmacy and near Bodia shop, a stone throw away from Pub Street)
Siem Reap
Tel. +855 (0)12 88 49 09, +855 (0)95 72 72 90
Grasshopper Bicycle Shop
House 586, St #26, Wat Bo Village, Siem Reap (opposite the popular Peace Cafe and just down the street from Angkor Village Hotel – One block back from the river)
Siem Reap
Tel. 016-337-363, 012-462-165
Open: daily 7am-6pm
see http://bicyclecambodia.com/ for map links.
Warning: I gave my bike to Camouflage Adventure for routine wheel truing and it came back with nothing done, so I got a refund. I would not recommend this shop, they also cleaned my bike with soapy water which I did not ask for.
This shop/rental place and Grasshopper sell some parts. Grasshopper has KMC chains.
But best place in town is on Samdech Tep Vong street, coming from the south of the city onto Sivatha road turn left (now heading West) onto Samdech Tep vong (with petrol station on corner)and go up no more than 500m, on your left there's the good bike shop. If you've walked up and road turns right, stop and go back.
Can't remember name of it but has all good Shimano components etc and trues wheels well, he even trues wheels for Camouflage Adventure.
I was just in siem reap and found 3 bicycle shops on route 6 (heading east out of the city centre) pass phsar leu market (on the same side of the road). I've also put it on foursquare with an exact map marker. Hope this helps. Cycle safe.