First, make sure your equipment supports IPv6 and is properly configured to handle IPv6 traffic. Along with the benefits of adding IPv6 to your network it can also introduce new problems so prepare for some common issues.

Use Hurricane Electric's World Report (seems to be auto-generated) and pull a report for the country you're in. Then compare ISPs that are available to you. The 'Adjacencies v6' column refers to how many other IPv6 connections to other companies they have (peering) which will improve IPv6 performance. The 'Routes v6' column refers to the part of their network that is available over IPv6. Both columns you can compare to the related v4 columns to get an idea how far along their IPv6 implementation is at. Now if the ISP has no v6 routes listed then they just don't offer any form of IPv6 to their customers currently. If the ISP has a very low ratio of v6 to v4 routes then they may have a limited IPv6 deployment where only available in IPv6 trials (e.g. limited to certain areas or types of technology used or even on an opt-in only basis) or just via Relay Services.

Some other things to consider when evaluating ISP's IPv6 offerings:

If you are a current customer of an ISP without native IPv6 connectivity then make sure to tell them of your interest in this. If no IPv6 options are offered by contacting who handles your account then make sure they send up your expressed interest/inquiry to someone with some decision making influence.

If your current ISP doesn't support native IPv6 and there aren't other reasonable IPv6 offerings from other ISPs in your area then in the interim you could consider:

Other useful resources:
Network World article: Finding an IPv6 ISP