You do not need to use Repels to catch a shiny Pokemon. Shiny Pokemon are just like any other and using Repel will force them away like any other Pokemon. So every time you use Repel or anything that drives back Pokemon, you are repelling the chance of encountering a shiny Pokemon.
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You could get a shiny Pokemon for crystal but you need luck to catch one ("_")
Yes it takes 1 out of 8000 chances to find a shiny Pokemon all you need to do is look for wild Pokemon for a long time eventually you will be lucky. Trust me when i say you can catch a shiny Pokemon because i have done it myself it happened to be a shiny shuppet which i found on the route west of lilycove city.
To get a shiny Dratini in Pokemon Crystal you need to find one in the wild or hatch one. This is extremely rare though with a 1 in 8192 chance of the wild or hatched Dratini being a shiny. There is no way in Pokemon Crystal to improve these odds.
To get a shiny teddiursa would be brilliant. It is very hard to find. The best way to get a shiny is through chaining. You need to beat the Elite 4 and get PokeRadar.
To get any Shiny Pokemon, you need: LUCK