There are quite a few different police uniform shops available online, many selling much the same range of products from 5.11, Arktis, Blackhawk, Under Armour etc etc. and very often at similar prices (do keep an eye out for sporadic promotions though as you can save significant amounts with Newsletter special offers or Facebook deals when they come up).
One of the best police uniform shops is PatrolStore.com, offering one of the best product ranges and probably consistently the best prices on the web (even before some of the fantastic special offers they often run). The site offers some great features which really help when shopping for police uniform supplies, top of my list being the large 1000*1000 good quality pictures. The pictures normally show the uniform items from several different angles and pick out special features where applicable to highlight them. A great example from their range of uniform supplies is their police vests, especially the Op. Zulu Advanced Tactical Duty Vest which shows a 1000*1000 front and rear view and then also seven additional pictures of key features, such as pockets, straps, docks and epaulette holders.
Speaking of epaulettes these are an often hard to find piece of police uniform which can be a real struggle to convince stores to release extra pairs if you loose yours or have several layers on and have not been issued with enough to have a pair on every layer.
Less important these days for the new "tactical" police uniform is a clip on tie, these used to be an essential piece of uniform, but the new moisture wicking polo shirts have replaced the traditional white shirt and a tie is no longer normally worn by police officers. (It is still a key bit of kit for your security uniform though if that's your industry).
So whatever your uniform (police, military or security) needs are check out the PatrolStore.com for all your law enforcement shopping needs.