Link works for me, check your browser security settings.

Yeah, it looks like their security certificate expired so many browsers block due to risk.

Here is their contact info:Email:

This email address is being protected from spambots. You need JavaScript enabled to view it.

Mailing/Shipping Address:

Oldskoolmx
8776 E. Shea Blvd.   B3A-205
Scottsdale,  AZ   85260
USA

Oldskoolmx.com seems to be long gone. Web site still functions but no reply and no product. Google earth shows an empty store front in a strip mall.

Check out Partzilla's list of other bikes the cap fits...  many YZ's, TT's, XT's, TY175, etc.  Many of those models are popular and someone might make billet caps for them. 

I remember seeing several billet caps for the YZ, TT, XT, and TY175 on ebay years ago.  Seemingly nowhere to be found now but i didn't look at every vendor that specializes in those models.  But the good news is OEM and aftermarket plastic caps are readily available from many sources a lot cheaper than from Yamaha. 

(I owned a couple bikes with that cap for several years and never had one shrink...  I know it happens though but a plastic one should last quite a while.  The DT400 I just sold looked to have the original cap and I had the bike about 6 years, cap was fine still)