I use stressfreeprinting. They charge around £150 for 20000 A6 full colour gloss leaflets including delivery.
A few things to bear in mind when thinking about leafleting;
Make up about a thousand of your own (using your home printer) and deliver them yourself to see how many replies you get. On average I now get 6 new jobs per 1000 leaflets delivered, having tested a couple of different designs. If your happy with the results then get a professional printer to print you up a batch. The larger the quantity you order, the cheaper each leaflet will be prorata.
Keep you ad simple, and check for spelling mistakes.
Now for the best bit - how to get your leaflets printed and delivered for FREE;
Loads of people use leaflets to advertise their businesses - Estate agents, take-aways, tradesman etc and they are always looking for reliable distributors. Most will generally pay around £40 per 1000 leaflets delivered.
I contact local businesses and tell them about my WC business and ask them if they would like to share the distribution alongside my leaflets. When I have 2 other businesses interested in this service, I then hire a couple of leafleters who I pay £60/ 1000 to deliver all 3 leaflets. I also give the distributors an extra £5 for each new sign-up (as an extra incentive)
Therefore the 2 businesses pay me £80 (2x£40). The £20 extra leftover pays for my printing costs.