I'm a purely ecom business. I don't want to buy and keep inventory but not all brands offer dropshipping options, what's a middle ground to achieve?
You can use something like Amazon FBA where you send all your inventory to one of their many warehouses around the world and they will securely store it for you. Then, when you have a sale, they will pack and ship the order to the customer for you. They even handle things like returns and customer service. I would recommend checking it out.
Create contact with unique distributor in your country that can drop ship for you while offering products not available on Aliexpress.
The middle ground here probably still requires the initial purchase of stock from your side as if it isn't being drop shipped then someone needs to hold the stock, unless you work on a just in time system. However, once this has been achieved you can create your own fulfillment and storage network delivered by a 3rd party that would prevent you from being impacted by some retailers not offering drop shipping. A 3rd party relationship with a business who can store your stock as consignment stock and deliver it to your customers may cost you some margin but it will save you from the hassle of holding stock yourself etc. So my recommendation here is sort of like an Amazon FBA model, except you replace Amazon logistics with a more affordable and smaller logistics partner to keep costs down.
Look for suppliers in your country of sale that accept dropshipping.
Look at the brands your competitors sell and reach out to them directly.
You don't need to sell all the brands in a niche, but you will want a really great collection of solid brands.
You could also start by finding smaller local manufacturers and asking if they will dropship custom products for you. Most will say no, but you really only need 1 yes for this model.
You might need to buy in advance, but you won't need to worry about warehousing.