im looking to buy a Downpipe for my T28 turbo, I have cash in hand

 

downpipe depends on the kind of manifold you have, and 1.6 or 1.8L and na or nb,

no such thing as a "universal" t25 downpipe.

 

its just an ebay manifold on a 1.8 NB

 

I didnt know eBay manufactures a turbo manifold for a Miata

again, you need to be specific WHICH manifold you have.

or dont.

then buy a downpipe that doesnt fit, and waste money.

 

... then you'll need an eBay downpipe that matches it and the turbo (if you're lucky you may find one that works- only then will you find out a few weeks/months later that your eBay "shiny" manifold has cracked and needs replacing).

Don't throw good money after bad money, you can't install a cheap+reliable turbo unless you weld it yourself and know what you're doing.

You'll save yourself time, money and grief if you do it right the first time which is easy these days with proven manifold+downpipe kits from Shuiend, Kraken, TSE or FM.
While these kits may be more than you want to spend initially, you wont be wasting your money trying to piece a system together from cheap parts that will be replaced with one of these kits in the long run anyway.

If you can't afford one of these kits and don't weld yourself, you shouldn't be turbocharging your car because you'll just be wasting your money and your car will be out of action for long periods.