The ones that use screws need that (metal/plastic) back plate which can do not always fit all soc 775 boards..from my experience.
If you are talking about the ones with push pins you can still buy them from ebay and other sites.
One thing to keep in mind, and am sure you know, is that the higher end ones for i7's were taller and have copper core like these: https://www.ebay.com/itm/226829150192
not like these for i3 and lower end i5: https://www.ebay.com/itm/234293654673
just examples...

What kind of a cooler are you looking for? Like a high end one or just something that's gonna look good? I'm not sure how it's like in your part of the globe, but in my country there's tons of Zalman, CoolerMaster and Scythe coolers on the second hand market for real cheap. The absolute best air cooler that you can get would probably be a Noctua NH-D15 with an additional LGA775 mounting kit, but more commonly you can get their NH-D14 which should support LGA775 from the factory. The other prolific line of dual tower coolers for LGA775 are the various Thermalright's Silver Arrow coolers. If you're just gonna run dual cores and not quad cores, a twin tower arrangement would be overkill. Some great single tower coolers that you can consider to get are Thermalright's TRUE(and it's various more modern derivatives like the True Spirit 120m), Prolimatech's Megahalems, Thermaltake's Frio series or Cooler Master's Hyper 212+/212 Evo. You can even get some water cooling with Corsair's original line of AIO coolers(H50, H70 and maybe H80), but you'd better be ready to perform some maintenance on them.

Aliexpress has a bunch of new AMD style coolers that come with LGA plastic adapters* compatible with 775.

OR get this - https://www.thermalright.com/product/true-universal-btk + any Thermalright cooler compatible with it.

*They are compatible with a bunch of old AMD socket 939/AM2 coolers too, if heatpipes are not protruding too much from sides.

The main limiting factors when choosing cooler are the motherboard and case:

1. The LGA775 motherboard must have enough space to install the cooler mount.
Any obstacles make choosing cooler much more difficult.

In my case with ASUS P5LD2, VRM heatsink is too close to the socket, which blocks the cooler's clamping plate.

2. On some motherboards the LGA775 socket is too close to the edge.

Big cooler with 120mm or 140/150mm fan will overlap the motherboard's dimensions and cannot be installed in a standard case.

In this case, you will have to choose compact cooler (with 92mm fan) or replace the PC case to a more spacious one.