This list is going to be ranked in order with the sportsbooks that have published odds on a regular basis near the top.
5Dimes Casino & Sportsbook (US Friendly)
The 5Dimes sportsbook has offered WWE pay-per-view for many years and I fully expect the company to have Extreme Rules 2014 matches available to bet on in May. When available the match markets can be found under the 'Other Sports' category on the betting board at 5Dimes.eu
The Irish bookmaking company, Paddy Power, have also offered World Wrestling Entertainment pay-per-view betting markets each month. I expect them to continue this trend with Extreme Rules.
When the odds are posted, you will find them in the 'Sports Novelties' section of PaddyPower.com.
Sky Bet is another company that has offered WWE wagering on a monthly basis. I would be very surprised if the time comes and there aren't any markets open for Extreme Rules. When they are available, you will find them under 'Wrestling' on the menu at SkyBet.com
YouWin have been offering WWE match markets on its website for quite some time now. The only recent event I can remember them missing is Elimination Chamber in February. If they do end up pricing up odds for Extreme Rules, you'll find them in the 'Specials' section of YouWin.com
UWin is the Canadian-facing partner of YouWin. As with its partner, the UWin WWE odds will be found under 'Specials'. The website address is UWin.com
William Hill is another company that opens World Wrestling Entertainment wagers on a regular basis. As with YouWin, William Hill did miss out on Elimination Chamber but did have odds available for WrestleMania XXX. It is worth noting that every time William Hill have WWE markets available to customers, the odds are taken down overnight so if you are looking for them late at night, they might not be there. It's best to check early in the daytime. When published, the odds are found under 'TV/Specials' on the sports betting menu at WilliamHill.com
Manchester-based Betfred opened wrestling betting for the Royal Rumble in January and WrestleMania XXX on April 6th. If the company decides to open a book for Extreme Rules, the odds will be available under 'TV Show / Special' on the sports betting side of Betfred.com
The sportsbook on Betfair also offered odds for the most recent WrestleMania. I am hoping the company decides to allow its customers to gamble on Extreme Rules because they came out with some intriguing prop bets for WrestleMania. For example, they set an over/under on how many times Hulk Hogan would say 'Brother'. If you are a customer of Betfair, please get in touch with them and tell them you would like to see more WWE wagering opportunities. If not - JOIN NOW and then tell them!
If WWE markets are opened at Betfair, they'll be found under 'Special Bets' of the regular sports betting menu of Betfair.com
Coral Bookmakers told me that they only open WWE betting markets for the big events like SummerSlam, Royal Rumble and WrestleMania. If they do decide to go against this plan and open a book for Extreme Rules, they'll be available under 'TV Specials' on the Non-sports A-Z' menu of Coral.co.uk
Bovada (US Friendly)
Bovada usually only posts betting odds for the big main events but it's worth having a look to see if they decide to offer WWE markets for smaller PPVs after one of the spokespersons for the company said they will consider WWE betting in future after receiving press about WrestleMania XXX wagers. If they are posted, you will find Extreme Rules betting odds under 'Other Sports' on the sportsbook menu at Bovada.lv
The Canadian-facing Bodog has WrestleMania XXX odds posted on its website. Just like the company above, I think it will depend on how much popularity and feedback the company gets following the WrestleMania markets. If they do end up being posted, WWE odds will be found under 'Other Sports' on the sports menu at Bodog.eu
I hope this post has helped you in any way. If you have only come across WWE sports betting through WrestleMania and would like any of the companies you have joined to continue offering WWE betting markets, I suggest you get in touch with them and let them know what you want. If there is demand, I am sure they'll supply.
Best of luck with your bets.