There are some various ways to look at this topic, of course.
I remember reading I think in the 2E Dwarf Handbook that they prefered to actually use items/tools/weapons for their clerics holy symbols. They didn't old up a little symbol of a hammer, they held up the whole hammer! That would depend greatly on the DM and how the 'church' they work for treats the holy item issue.
After 3E came out, I decided to treat it as an item 'upgrade'. You can Masterwork basically any item in the game. A masterwork crowbar is +2 to any rolls using the crowbar. It costs +50 gp on top of the items base price (except for masterwork thieves tools).
I treaded the holy symbol/item the same way. If you wanted a shield that could act as the holy symbol, you had to order it done, and add the cost of the holy symbol on top of it. This can be done to any shield, weapon, armour, or even other items. I also like this because it actually makes priestly vestments 'holy' for that religion/faith.
(Un)Holy Upgrade:
Any shield, armour, weapon, or other items within reason can be used as a holy symbol if they are made for that purpose at the items creation. This functions much like the bonuses for masterwork items. The item must be made under the same rules as any other holy symbol, and receives the same blessings. They are typically only avaible from the church (or sponsered shop) in question. Market Price +25 gp to base item cost
Characters should be able to afford this with ease after level 1. Heck, you can even stack it with Masterwork armour/shield. Other characters can buy it to, because it looks cool with disguises when you are trying to fake your way into a temple.
Example: a mighty +2 masterwork composite long bow, holy to Ehlonna
masterwork thieves tools, holy to Olidmmara (sp?)
It's cool turning undead with thieves picks....
