Affiliate marketing for bookmakers and casinos

Best way at the moment is to know something about the market or recruit someone who does. So many "£10 to £1000" Facebook groups with huge followings. Idea is to offer tips and build up to £1000. They don't tend to be wildly optimistic tips so they offer a decent enough return that people keep participating and new ones join. Anyway, you can push your affiliate links through the group or a dedicated community website.

A little while back I checked with sendgrid support to see if they allowed gambling (they are less restrictive than many ESPs) and they said...

Currently Sendgrid allows emails with gambling content under strict caution following our terms of service and policies listed here
Email Policy | SendGrid
Terms of Service | SendGrid

If you are going to do affiliate marketing, why pick a niche that is going to be so burdensome right from the start?

I've been doing affiliate / CPA marketing for over 16 years.

I heavily promote PPL (pay per lead...lead generation) offers. This is because there is no credit card / purchase required to complete an offer. All a user has to do is fill out a form, so conversion rates are typically much higher compared to offers that require a sale to be made.

Bottom line, it is far easier to get someone fill out a short form than to get them to pull out their credit card and make a purchase. So why struggle with trying to sell this or that, when you can provide free information that users want/need and get paid well doing it.

Online lead generation is a multi-billion dollar industry. Done right it can be far more profitable than many imagine. Everyone that I know that is in the business and knows what they are doing, for the most part, does 6-7 figures. While that may sound like a huge range, much comes down to one's abilities to scale and effectively build/manage the infrastructure needed to scale.

Didn't mean to go off rails on your post. It just seems like you are wanting to start out with the odds against you. Anyway, something to think about.

I would go with content marketing for this one. There are a lot of ways to market your website with content. Like for example, you may do ebooks or guides and then sell this on Amazon Kindle. You may promote the site through your books.

Another content marketing idea is to do guest posts on related niche sites.

You may also join related gambling forum communities and become an active member there.

Then of course, there is PPC. Like what others here have already siggested, you can do PPC to related niche websites as well. There are definitely a lot of these online and all you need to do is to get aa good email template done.