There's a great resource called the RCT2 Database, located
HERE.
Using the search feature I found one entry for a beach umbrella with a dat name of "beachumb", which was made by Spin, converted from Zoo Tycoon. Spin's site is no longer up.
Normally all the custom items used by a park designer should be included in the park file if they correctly checked the 'export scenery' box under options.
Usually we tell people that they need to contact the designer to obtain the file. Since it appears that Spin's site no longer exists, that would be difficult to do. I have the file if you will pm me where to send it to.