I can't tell you where you can find a Tamron 24-70 to hire, but I can tell you why you can't find one.
The first and most important reason is economics. Cheaper lenses are fundamentally less cost-effective to hire and are consequently less attractive.
Here's how it works. The Tamron 24-70 costs about £800 to buy and the Nikon 24-70 is about £1200, so the ratio is 2/3rds and you might expect the Tamron to cost about 2/3rds as much to hire. But you would be wrong. The hire price has to cover the cost of owning the lens, but also the costs of processing the hire booking - including communicating with the customer, packing, shipping, checking in, unpacking, cleaning, testing, etc. All these things are basically independent of the price of the lens, so they have a bigger proportionate effect on a cheap lens than on an expensive one.
As an example, at LensesForHire the Nikon 24-70 costs £55 for a long weekend (3 days) and £79 for a week. We don't stock the Tamron 24-70 but we do stock the Nikon 24-120 which costs about the same to buy as the Tamron. You might expect hiring it to cost 2/3rds of £55/£79, which would be £37/£53, but in fact - because of all those costs which affect all lenses equally - it's actually £43/£62. So hiring the Nikon 24-70 costs 27% more than the Tamron, whereas buying it costs £50% more. Advantage Nikon when you're hiring.
Plus we haven't considered postage. Most of our customers aren't local to Maidenhead so for them we need to factor in postage costs. That's about £20 for the round trip (or more if you need some sort of priority delivery), and again it affects both lenses equally. So for 3 days / 7 days the Tamron is £63/£82 including postage, and the Nikon is £75/£95. Therefore the Nikon costs 16-19% more to hire, but it's still a 50% more expensive lens and it's therefore much more attractive to hire.
The second reason is reliability. By and large, OEM lenses tend to be better built than those from the independent manufacturers; and the Canon/Nikon service departments are *much* quicker at fixing problems. So as the owner of the business I'm going to expect my Canon/Nikon lenses to be in service for a higher proportion of the year than the Tamron/Sigma lenses. Obviously that's hugely important.
None of this means we won't stock third-party lenses. We will if they occupy niches which the OEMs don't. For example we have the Sigma 120-300 f/2.8 which has no OEM equivalent; and we have some of the new Tamron 150-600s on order. But where a third-party lens competes head-on with an OEM lens, we'll tend only to stock the OEM version for the reasons outlined above.
Hope that makes sense. Happy to discuss.