There are three types of sponsorship deals:
Commissions: You place an ad for free. If someone purchases via your link, you get a commission. Such deals are good for affiliate marketing on YouTube (or with huge distributions) but don't work for early-stage newsletters. Avoid them at all costs because you're an early-stage creator with little to no authority to sell high-commission deals.
Pay-per-click: You place the ad for free and get paid every time your readers click on the ads. This is the model my newsletters currently operate on (and most early-stage newsletters do the same). It's risk-free, and r/beehiiv does an incredible job with sponsorship opportunities, even if you are under 100 subs.
Slot bookings: This is the ideal situation. You get paid to place an ad, and advertisers decide based on your previous data and your relationship with your audience. There is no pressure on your end to perform, but you only sell what you believe are good products for your readers (ideally). If your readers trust you and your advertiser's product is good, it more or less gives good returns.
How to get these deals?
If you're going down the affiliate path, visit relevant websites and check if they have a referral/affiliate program. For PPC, Beehiiv's paid plans are my go-to. To get slot bookings, you must grow beyond 3000 subscribers based on your niche.
PS: This is an estimate based on the brands I have worked with and read about, not an absolute metric.